update:
i call electrician and he tells me that martin should not have wired the shower into the 15 amp plug like he did and that cable was too thin.
he has rewired it into the fusebox and everything works ok (although he says the house needs rewiring sometime next year as the sockets are obsolete(?))
not happy as it cost me £180 on top of the money i gave to martin to install the shower but there you goes.
thanks for the advise everyone.
harry (mr)