Ed Sirett wrote:

snip


When you had the cold water off air may have got into the Hot pipework.
If you have a 'dead' branch somewhere this could harbour some air. The
effect of the air would be for a small flow to occur through the boiler
when the cold taps are worked.

This boiler is able to dump residual heat after doing HW into the CH
pipework so this does not precluded the fitting of a non-return valve (NRV).

However since this boiler did work correctly before searching for the
trapped air is likely to be more fruitful. A NRV would solve problems. A
shock arrestor won't help IMHO.

I have just run water through the 'dead' branch I added in preparation
for the en-suite I need to put in (sorry, didn't mention that in my
original post). There was indeed air in the pipe, and getting rid of
this seems to have cured the problem. Hope it stays away now. And many
thanks for all the suggestions!

Johan