On 30/11/2019 19:30, The Other John wrote:
In article 6,
says...
Any ideas?

Do you live in a hard water area? Limescale is a possibility.

Agreed.

Make? I've been amazed how reliable my non-thermostatic non-bar type has
been in hard Cotswold water (probably Screwfix). I believe it has
ceramic disks.