I have been showing my 13 month old since he came of age at 6 months and this has never presented itself before. Lately - the past two or three shows - when moved at a trot he initially seems fine, then seems to go down on his rear left leg, later that day he's back to normal; The vet gave him a full check over, she cannot find anything wrong with his rear end and believes the problem to be related to how fast he's growing (something about his muscles having to catch up?) I have booked him into hydrotherapy to try and improve his rear muscles and the vet says she will x-ray him if I'm still worried - I've booked it, but would like to hear your thoughts on it before I subject him to anaesthesia because of my hypercondria. Any advice would be greatly appreciated...

Personally I'd rather know what's actually going on before you start anything physical like hydro - you may inadvertently make it worse.

Definitely diagnose first (my opinion).   Just a thought re the showing - do you take him straight off is bench and into the ring - could it be he needs a little 'warming up' first?  

I'd certainly go for x-ray if nothing shows up with simple palpation.

Another vote for accurate diagnosis first. As he is over 12mth hip X ray could be sent for BVA hip score as well so worth the effort IMO

I have been informed that doing hydrotherapy with an existing problem could make it worse as the dog will still use the leg in the way it wants or not at all.
Definitely get diagnosis first
Look up the SMART clinic, brilliant place.