well with the shower running hot and cold, if it did it on all the hot water taps too i would have said the hot water thermister is faulty but if its only the shower then it may be the mixer valve on the shower at fault, they can cause terrible problems with combis as can washing machines powered off them too.
as for your banging pipes i suspect it may be a loose jumper washer on the main stop ****, get a plumber in.
and terambulan, you need to try a different plumber !