I have a 12 week black lab dog that was homed at 9 weeks (Dam was a  very good natured family pet) and presumably should have picked up acceptable behaviours like bite inhibition. He mouths/bites/pulls trouser legs and sleeves/humps a lot of the time and though we have tried to stop him firmly but gently his behaviour is not improving. We have even tried holding him down as recommended to establish dominance which quietens him short term but he's biting again 10 mins later. Also, I gather that labs of this age should not be walked but he appears desperate for exercise (though we are both at home playing with him) and a run is the only thing that seems to tire him out; he does sleep a lot but when awake is a real handful. Please help - he's a beautiful dog but we feel at our wits' end...

Bite inhibition is something that you, as owners, must continue for quite some time.  It is perfectly normal puppy behaviour as they, like babies, explore the world through their mouths and test responses.  Ian Dunbar has an excellent article on bite inhibition in his book Teaching a new dog old tricks; highly recommend.  Humping has nothing to do with sexual behaviour or dominance, is practised by both bitch and dog puppies and is just an example of "excitable" behaviour; just walk away or redirect him on to something more acceptable like a tuggy toy etc.

Have you tried stuffing kongs with food, or a buster cube with titbits which will keep him mentally occupied; if he is fed complete you could also try placing it in the grass so that he has to "hunt" for it rather than just in a bowl.

Just a few ideas for you.

Tohme has given you good advice there. Just wanted to point out that holding a pup down (bit like a junior version of an alpha roll) is not a good idea. At the very least it is annoying and upsetting to the pup. Distraction/redirection is definitely the key - and a Kong/chew toy is ideal for this :)

Just to say thanks so much for everyone who has taken the time to reply - a very positive experience all round and makes us feel supported! We are going to take him to obedience classes soon but are already doing basic training.