Just wanted some advice.  Sam is now six months old and is turning into a right old Kevin. When he is off his lead having a run around, and I see another dog - I put him back on the lead.  He is a really excitable, jumpy lab who loves other dogs.  But am I doing the right thing putting the lead on or should I just let him off the lead.  Recently had a bad experience where sam was pinned down by another dog - the other dog owners just walked by and didnt pay any attention - they just laughed and said oh, come on (name of dog) the dog didnt listen and I had to pull the other dog off.

Jenny & Sam

If you cannot recall your dog with reliability and you do not know the other dog why would you let it off in potential danger.  If your excitable, jumpy dog who "loves" other dogs approached my two males whilst they were on the lead your dog could be badly bitten; do you want to put your dog at risk?  Work on your recall several times a day from now until at least 18 months if not longer, use a trailing line so that he cannot escape and only let your dog off in areas which are safe eg where he cannot run into a road or a somewhat less than amused dog (or owner).

If you always put him on lead when you see other dogs then he will become very frustrated at not being allowed to play and socialise and you may find that his recall will go downhill too.  If you do not allow him to freely interact and have another dog tell him off occasionally then he will never learn any manners.
Sally
Edited to say freely interact with dogs that are off lead only I should have said. :)

Hi Jenny, you can use a long line or flexilead whichever you prefer to keep Sam out of trouble, so he knows how ever far away you are he must obey a call to you. Another thought is do you play/train him on walks rather than leave him to do his own thing and rush off to other dogs? Recall is assisted by you being the most fun person on the planet and source of all good things to your dog. These are from my own painful experience as I didnt realise the importance of getting Morse focused on me when I got him at 6 months and allowed him to run around with other dogs too much. Now whenever we go out I have a plan, time with other dogs is strictly limited and if there is a slight hesitation in a recall its back to the line!

Once pups are 6 months older dogs will put them firmly in place if they rush over, and some cant be bothered with youngsters. We taught Morse to down when  other dogs approached.This saved his hide and gave me a chance to intervene if necessary. He still does it to show them hes no threat and waits for the other dog to signal its intentions. HTH