how can i fix ravenheat LS100 combi boiler?

my Ravenheat combi LS100 would not give hot water without turning the heating on. we put up with this until the weather got warmer. the diverter valve was changed (had split in rubber) but then it wouldnt start up at all. i was told by the engineer that the pcb was faulty so had it changed, still as dead as a doe doe. any ideas? the green on of button does not illuminate when switched on but im not sure it is meant to. Help!!! engineer gave up and has done a bunk after breaking the water pressure gauge which cost me another 45quid....
Hue.......