Bert Coules wrote:
Brian Gaff wrote:

Aha, so could you have some blockage in the water softener?

Thanks for the thought. It's possible I suppose but would that cause
the effect I'm seeing?

Also, the same softener feeds two other taps, two cisterns, a shower
and a washing machine and I've never noticed any problems with any of
them.

Indeed!
It is only the tap over the wash hand basin that does this. I've never seen
the bath, shower or the kitchen tap do this.
We live in a soft water area.