I have a brindle 12 month old staff puppy who has had 1 2 1 training with a puppy training which he behaves excellently for, however, once the trainer has gone he wont listen to even after months and months persistent training.

He keeps nipping and darting around like a loony still.

On another point i think he looks as if he has not grown since he was 9 months old as he is very small and not very stocky.  Do some staffies stop growing then start again after a few months?

>I have a brindle 12 month old staff puppy who has had 1 2 1 training with a puppy training which he behaves >excellently for, however, once the trainer has gone he wont listen to even after months and months persistent >training.

Do you mean that the trainer has trained your dog? or has trained you and your dog together?  from your post it's not really clear  

If the former then I'm afraid that's the usual result :(  YOU really need to be doing the training with your dog for it to be successful.   I'm surprised that the trainer hasn't taught you how to control (NOT STOP!) the mouthing and nipping, it's one of the first things they should have taught you 

A really good article on understanding mouthing and nipping and how to control it, is here http://www.jersey.net/~mountaindog/berner1/bitestop.htm  the method given is very effective ;)

One thing to remember is that at just 12 months your dog is not an adult and won't be for some time, you can expect puppy and teenage behaviour from about 12 months ;)  for a while it will seem as if all the training was for nothing, but if you keep up the training then it will come together as your dog matures.  Remember as a terrier he will need to be given brain exercises as well as body exercise ;) try some 'find it games' with him, he'll love that.

Regarding his growth, staffies aren't supposed to be very large dogs, according to the breed standard 14"-16" and 28-36 lbs (many staffies I see outside of the shows are far too big, both in height and weight ;) ) and at 12 months your dog won't have started to fill out yet, this will come with maturity ;)  is he really smaller than 14" ?

Staffords can be strong willed dogs when they choose, so you do need to nip the nipping in the bud, so to speak. 
As for his size, exactly as Dill has already said.  He's only 12 months old, both my staffords took a long while to look anything like the breed standard.  But I would take that a good thing personally, slower to grow and mature in body is probably better for the animal.

Definately agree with Tracey- be firm with him and dont allow him to mess about whether its playful or not- he's of an age now where he shouldnt be doing this

staff puppy who has had 1 2 1 training with a puppy training which he behaves excellently for, however, once the trainer has gone he wont listen

Must admit it does sound as though the trainer is training the dog as opposed to yourself from the way this is worded. Many of us have an air of authority about us and find dog training easy, I dare say that the trainer also has a no nonesense appeal that your dog knows he must adhere to, dogs know who they can twist around their little 'paws'. :-)

How has your trainer been training the dog?

The best trainers will use reward based training as anyone can do that and learn it effectively, and that is what you should be doing to make your dog understand commands, once he is obediant as he is learning he gets a treat when he does as you ask, and also learning to trust you as his friend and provider then you need to implement strong eye contact and strengthen your voice when he is wishing to do his own thing.

When he is not compliant he needs to go into time out, behind a dog gate in a crate away from you and the rest of the household for a few minutes, in time, with reward, firm instruction and time outs he will get the message and be on his way to doing exactly as you ask. :-)