Why is my water still to hot?

Previous Q & A :-
I have a recently installed worcester 24i (LPG). The tap water gets very hot and after a while runs cold, read it should be limited to 55*?

your heat exchanger may be scaled up, especially if in a hard water area. either that or the dhw sensor is faulty.

Had the worcester engineer out. He changed the dhw sensor and set the boiler up correctly (said it hadn't been). He told us it should now be ok, but mentioned that if not it would be because the incoming water pressure is to low and there was not much we could do about that. Needless to say the problem has not been resolved. We don't live in a hard water area and it's a new boiler so can't see the heat exchanger being scaled up. Is the engineer right about the water pressure or could there be another fault?