Well I'm just back from the dealer and the start stop is now working perfectly. New battery and IBS, plus they've fitted a new air con pipe and that now works better too!
I guess they must have reset the trip computer aswell because the best av mpg I was getting before today was 37.5 and its already shot up to 43.8 and is still going up!
RESULT
Thanks for all the tips!
MY TA punto is in need of similar I presume,
I've done terminal cleans while the battery is being charged--still no better,
Charlie - Didcot