I have been away and had time to think of questions I need to ask.

I walk my dog on a school field that backs on to a pubs garden which has 2 "pub dogs" Staffy and a Jack Russel.  My 9 month old dog approaches these dogs (which are securely behind a fence due to them backing onto a primary school) they go ballistic and snap and growl and throw themselves against the fence, Arthur however doesnt seem to understand their hostility and will wag his tail like mad and not come away until I distract him.  He doesnt seem to have any fear is this because he doesnt understand doggie language or because he is young and nieve?   Also am I right in letting him approach these dogs?

I'd suggest he knows they can't get to him and is simply winding them up.  I wouldn't let my dog do it, no.

I agree with Digger, if your dog happens to meet these dogs whilst they were out there could be a problem.

yeah, I do feel for these dogs because I dont think they get walked.  It does seem the more they see him the less aggressive they are getting.  The staffy has started wagging its tail and barking but not snapping.  But I will still keep taking him away if he gets too close to them. 

I know alot of dogs that are like that behind fences , once they meet face to face they're excellent and just as playful. I wouldn't worry about it unless these dogs are known agressors.