I think I've finally cracked the distraction of other people and our puppy even ignored a group of 3 joggers tonight while I told him to stay - even though they were shouting "well done!" to him as they passed. 
However, will he leave strange dogs on their lead alone? Like hell he will. If he is on his lead I can have him ignore other dogs if they are not showing signs of wanting to greet him, but if he is off lead, no amount persuasion will keep him away from them. I do put him back on lead if I see an on lead dog approaching, but every once in a while he spots one first and that's it- he's off. I don't want him to get hurt! Plus I want him to learn good manners!
How do you safely train them?