Thank you for all the replies. Hope everyone had a good one. I'm begining too think the ring main might be iffy. Although i've recently had a new CU fitted and everything seemed ok. There certainly seems to be something leaking back through the neutral.
It is not your ring which is at fault, neither is it something leaking back through the neutral.
Unfortunately until now you've been advised by plumbers, and despite what they like to think, they don't actually undestand electrical installations.
Here's a really easy thing for you to try.
Replace the tank stat with a bog standard light switch. Retest your circuit. If the problem has gone away then it's a faulty stat. If the problem remains, then it's time to start investigating further.