Pete C wrote:

lots of useful suggestions snipped

Many thanks for that Pete, I'll certainly look into these.

OTOH there are some inexpensive mixers on Ebay and one from
Toolstation with 5yr guarentee costing £60 so it may not be worth
fixing it. Even if you can obtain an original replacement it may
suffer from the same problem again.

Followup q: is the separation of the H and C pipes on a shower mixer bar
standard? Having only finished retiling the shower a few weeks back I
think I might spontaneously combust if faced with changing the mixer for
one of different dimensions...

David