Worcester Bosch Highflow 400 - no hot water?

All you have described indicates a blockage/partial in the domestic H/E.
The little HW when the CH is on is quite common, they use a thermal store to provide heat to both so if you have the CH on, you deplete the availability of the store to provide HW.
The pilot going out is possibly due to that blockage, the boiler overheats and the interrupter in the thermocouple puts the pilot out.

These boilers were a fine idea, but they literally are a 'washing machine' sized boiler that didn't meet the concept, over-complicated in design (my opinion only).

Ditch the damn thing and fit a Vailliant 937 (also a thermal store) good water flow and more reliable.

Carefree
July 2010