HOW CAN I FIX MY PERFOMA 28 I?

If i remember correctly this model has a pre-heat facility and this is usually indicated by a flashing light at the hot water symbol. You say the neon isn't flashing but have you tried to switch the pre- heat function off?
If you have and it is still running on it is probably the micro switch on the water flow switch sticking in the run position after the tap switches off.
The book should tell you how to identify the right switch..

T.h.s
June 2007