Worester Bosch were very good. responded to my email within half an
hour (I was most surprised - I usually despair when I have to rely on
organisations replying to emails, particularly urgent ones ...).

I managed to book a next day "tiered price" appointment - ie one price
if it just needs service, another if it needs any parts, and a third
price if the heat exchanger needs replacing. Not unreasonable,
particularly when it's being serviced by trained Bosch people, and
nice to know the cost is capped.

He sorted out a long standing problem which I posted about a few years
back - when the central heating temperature dial is close to the
actual temperature, for low settings, the boiler repeatedly fires up
then shuts down instantly, over and over again. For the benefit of
anyone else who has this problem, he replaced the main controller
board.

He also replaced the heat exchanger plate, which was the source of the
leak.

However, in terms of hot water not firing up, he said the boiler
seemed fine and that he thought the problem was cross-flow in a mixer
valve, presumably high pressure cold water leaking through a mixer
into the hot water inlet.

Now, I know that my shower does have this problem, but am not sure why
the boiler is suddenyl unable to cope with it after five years.
Alternatively, it could be a new mixer fault elsewhere.

Does anyone have any ideas on how to easily diagnose the source of
such a mixer problem - I have four sets of mixers - without physically
disconnecting each pair of taps in turn and trying the hot water until
it works?

Would I be right in thinking that the more modern taps, such as a
4-year-old kitchen tap, which have physically separate outlets for the
hot and cold streams in the tap head, cannot be the source of such
problems?

Hope all this helps someone in the future ...
Thanks,
Richard