Thanks for the all the replies.

I have searched for the boiler model underneath but can't find it.

However, I am pretty sure that it is a VCW242 or 282?? The only
reason is because the boiler engineer accidentally left the manual and
your description of VWC models fits ours exactly.

I am also pretty sure that the 'diaphram' is the one that you are all
describing.

Therefore, any ideas why twice in 10 days the hot water stopped, only
for it to come back again later on in the day. When it failed, the
pressure / flow rate through the ho****er tap was a lot less than
usual.

Many thanks