How can I make my dog be less focused on other dogs when out on a walk (and at training?)  He is a well behaved 21 month old GSD (normally) but as soon as he sees another dog out on a walk he will go straight over to them, says hello and if we call then he comes straight back (at which point he gets a reward but will sometimes go straight back to the other dog after).
It is starting to affect some of his other training when other dogs are working off lead at the same time as him when he will be fine for a lot of the time but will sometimes dart off in the middle of an exercise to say hello to another dog.

I don't know how to deal with this.  We are going to start going to a park in the next town (as where we normally walk is fairly quiet) and try and overload him with seeing dogs so that they just become normal but I don't know whether I should be letting him go to another dog and get it out of his system; stop him from going over completely by playing etc and being more interesting; or let him think about going over to one, telling him off (or using a remote spray collar) and then rewarding and playing when he stays.

You've obviosly socialised him well with other dogs and it's great that he like them.  Please make sure it stays that way and don't use spray collars or other aversives on him that may make him associate the other dogs with the punishment.  The fact that you can call him away is great and shows that he can respond even when he is trying to interact with the other dogs so it really is just going to be down to more training and management.  I would keep working in the less distracting park until he is reliable there rather than over loaing him as he will likely just find it more difficult to concentrate. 

What I would do is start taking him out on a harness and long line so that he doesn't get the reward of running over to the other dog first.  He can still have some freedon with the long line.  Use the other dogs as a reward, if he recalls to you ask the owner if he can say hello and take him over.  If he doesn't then go straight on past or go in the other direction.  Try using really high value rewards when you are trying to get his focus when other dogs are about.  Make walks about training and focusing on you, which needs to be fun.  Does he loves toys, for example a game of tuggy?  Get him really focussed on a tuggy toy and keep it for using on walks and when there ar other dogs about.  Maybe make walks a mix of doing basic obedience, setting trails for him to find, playing tuggy etc.  Often youngsters at this age are easily distracted and find it hard to focus and if they are left to do their own thing on walks for any period of time they will start entertaining themselves, so it's important to keep them occupied and associating you with fun stuff.

I would also keep him on a longline or long lead when you are doing exercises at class aswell, maybe have a word with your trainer - he/she really should be advising you on this. :-)

I will get out the long line (thought we had done with it :-) )again.

As you say his recall is good and we are pleased with that but I want him to check in (or even not want to go and see) with use before he goes over to the other dog.  Do you think we need to put him on the long line, then when he sees another dog call him to us and reward and then build to only rewarding when he doesn't try and rush off as soon as he sees them?

Have to say I agree with the above about not using a spray collar or anything similar.  It sounds like you have a well socialised dog and you don't want to start reprimanding him for being friendly.

Have you taught him 'stop' or 'wait' on command?  My male Rottie loves playing with any dog but I don't want him charging up to anyone so I have taught him 'wait' on command.  I then collect him and we walk over to the other dogs so he still gets a reward for doing as I've asked.

Long line.

It's the only way to get those commands rock solid so they become automatic.

At the moment, though he wants to please you, he is also keen to please himself, and he is exercising his choices! The long line allows you to ensure he makes the right choice. You can gently encourage him to always return to you, where he is rewarded with praise and the odd treat. Use tracking and scent work to make being on the line interesting and rewarding for him.

Yes it is trying and does require loads of patience, but, you'll get there. Absolutely no need to use spray collars etc.. and could be disastrous.

He knows stop on recall and wait (as an obedience exercise) and we can try those out on a walk.

At training it is a slightly different story as he is normally off running an exercise and just peels off but we can start trying to use the wait command there too.

I would do as you say, but also ensure that a recall means he waits with you when you call him not just touch base and run off again.