Just a follow up and thanks to all that posted ideas and help on here,
Went round the system checking wires, connections etc and generally jiggling them to make sure all were secure. also turned down the pressure switch in the varistream.
Yesterday was a 'normal' day - no big cleans all done before and all was well.
Today was several big cleans 40 mins + then a gutter clean followed by 2 new cleans and the system went right through the whole day, not a single problem, when turning off the tap i no longer get burst joints and i'm sure the flow rate is far better than it was.
So i feel there may have been a dodgy connection somewhere.
Many thanks
Darran