On Sunday, 21 April 2019 11:15:29 UTC+1, Another John wrote:
He said his Mum used to bleach their cups on a regular basis. I'd heard
of this before, but I've never gone near the idea, for fear of poisoning
myself.

I use Lakeland Tea Away, which has been discontinued, and replaced with Astonish Cup Cleaner (contains: Sodium carbonate peroxide, Sodium Carbonate).

Basically a non-chlorine bleach.

Several applications interspersed with a plastic scourer may be neaded, as the bleach doesn't seem to penetrate tannin very far.

Owain