Bentley is doing great with his training, yet I have some questions...perhaps more for reassurance than anything else as this is my first dog.

1) When out walking, he will start to pull a little bit on the lead. It's not bad, not great tugging or pulling me off my feet but the leash is pretty taught. I say "heel" and he'll slow down a little bit...then go back to the pull. I'll give a little tug and repeat the heel and he'll walk next to me for a bit. I praise him and after a few steps I reward him with a little bit of cheese (he loves cheese!). As soon as he gets the cheese he thinks that's a release command and tries to pull away again. It's the same when we sit before crossing the road. Sit's happily, cheese and away he wants to go. I'm fairly certain its something i've done wrong somewhere??

2) Recall. On our own he's really good, will come when called and gets a treat, will come to the whistle too. If there's any distractions though he will rarely come to me. That's normal for a 5month old puppy right?

3) Stays. Getting him to sit stay is so hard. He always wants to lie down because he thinks that's what I want. I've not even tried to do it standing yet as i know he'll just want to sit and lie down. I'm thinking perhaps a check cord and starting on a "woah" command in my back garden?

4) Food guarding. For some reason he's started to growl a bit when I approach him while he's eating. It's strange because if i put the food in the bowl and stay next to him stroking him all the time he's happy. If I walk away for a min, then come back towards him and try to stroke him he'll growl. I've started to do it then when he growls a bit i'll drop a bit more food or a treat into his bowl, letting him know that i'm not going to take his food from him. I THINK he's getting better but some feedback would be nice?

5) Last one for now, I promise!! I WANT ANOTHER GSP!!! ok sorry, i'd like another dog in a while for him to have a buddy to play with. When would be a good time/age to look at getting one and is having two twice as hard as one, or just kinda the same?

Thank you for any answers you can provide.

So, here are my thoughts on your questions:

1) The cheese treat has become the release in the dogs mind.  For example, when you are at the corner you have the dog sit, give cheese, then start walking.  I would not treat in that situation since the dog already knows the drill.  I would have him sit, stay, wait various amounts of time, then say heel and walk.  Once you both start heeling then tell him good dog (or you could release him then and give him a treat).  He did the sequence right so he gets the verbal good dog letting him know he did good.  Same thing with the heeling.  Quit giving him the treats until he does something really good, like heel for several consecutive minutes, then release him with your release word and then treat him if you want too.  He is giving you the behavior that he thinks will get the treat.  Once he has the treat in his mind he is done and can go about business.  Just be aware of when you are treating and think about what the dog is thinking.

2) Recall. On our own he's really good, will come when called and gets a treat, will come to the whistle too. If there's any distractions though he will rarely come to me. That's normal for a 5month old puppy right?

Pretty much.  What this tells me is he is not ready to be off a check cord and needs allot more reps with the recall where you can enforce the command.  Once his is reliable in a controlled environment then add a distraction.  Once reliable there up the ante a little more.  If he fails at any point go back a step and work some more.  The recall is IMHO the hardest to get to reliable and takes thousands of training recalls in all kinds of different environments.  Be sure you never get angry or upset with him if you call and he takes some time coming.  If he comes to you even if he ran you around the neighborhood first, you have to praise him because he came.  He will not associated the correction with the running around, but he will think you are mad because he came.  Dogs think in the moment.

3) Stays. Again he doesn't know what you want.  Right now a leash is what you need and a quiet place.  Put him in the sit and stand next to him.  Wait a few seconds and release.  Praise.  Build on the time that way.  Once you can do that for say 30 seconds or so, then sit/stay and you step in front of him toe to toe.  Again, build the time.  Repeat moving gradually out to the end of the leash.  Once at the end of the leash and a full minute you can start working off leash and getting further away.  Stay without distractions for now.  If he lays down step in and pull straight up ont he leash and tell him to sit and stay again.  Be sure you praise too and be patient.  Eventually you will do this in various locations and add distractions, but make sure your dog is solid before you move to the next level.

4) Food guarding. For some reason he's started to growl a bit when I approach him while he's eating. It's strange because if i put the food in the bowl and stay next to him stroking him all the time he's happy. If I walk away for a min, then come back towards him and try to stroke him he'll growl. I've started to do it then when he growls a bit i'll drop a bit more food or a treat into his bowl, letting him know that i'm not going to take his food from him. I THINK he's getting better but some feedback would be nice?

That is the right approach for food guarding.  He needs to know when you hand is moving toward him and the food, good things will happen.  I would use something special and good when you add to his food.  I think I would drop the stroking for now and just move my hand in closer and gradually and if you get no growl add the treat.

As for the puppy roll from the other poster, that is an old method and most behaniorist today agree that it can and is often dangerous and has caused more bites than it fixes problems.  As will all things though, there can be situations where it can be used, but I would not use it with a GSP who is food guarding.  A good book for suggestions for dealing with behaviors like food aggression is "Aggression in Dogs" by Brenda Aloff. 

5) Last one for now, I promise!! I WANT ANOTHER GSP!!! ok sorry, i'd like another dog in a while for him to have a buddy to play with. When would be a good time/age to look at getting one and is having two twice as hard as one, or just kinda the same?

GSPs IMHO are happiest when they are around other GSPs.  So, if you want another one now is as good a time as any as long as you are ready.  As for work, it really isn't that much more.  I compete with all three of mine in conformation, obedience, agility, etc. so it up'ed the amount of training time for me.  It will also in your case double you food, boarding, vet bills, etc.  On the other side though, your dog will have a playmate and they can entertain each other.  Since I work from home it is really nice because the dogs go out in the yard in the morning and stay out and do stuff together until afternoon and they do not bother me while I am working.  On bad weather days they are in with me and can be a pain if there are too many bad weather days in a row. As for the dogs getting along, I have three (2 females and a male) and have had no problems.  My first two were females and I added the male several years later.  I will be adding another female in about 6 months or so since I am planning on breeding my middle girl and keeping a puppy.