Ariston Euro Combi - Hot Water problem?

Euro Combi SX 20 MFFICE (GC No: 4711602)

A few weeks agao, I had a problem with my heating - cured by changing the expansion vessel and installing a new PRV which was leaking (causing the pressure to drop). Heating & hot water were both working wonderfully well, until yesterday morning when I turned the hot water tap on ..... ghastly noises from the boiler and a rapid increase in temperature - causing the overheat cut-out to kick in. The temperature goes up to 140degC. When it cools down, I press the reset button & the boiler fires up, and the heating functions normally.

As soon as I try the hot water - the same thing happens again.

My gut feeling says it's the diverter valve - but any suggestions (or confirmation of my diagnosis!!) would be much appreciated.

Many thanks

Rob