My toilet is haunted... OK, not really, but it has a mind of its own.

[CointreauVersial]

Whenever we run the shower, the toilet suddenly starts to overflow, with water running noisily out of the cistern into the pan.

Once the shower has finished (and the tank in the loft has stopped filling up), the overflow also stops.

What's going on?!

it's possible that you have one of the modern quiet filling valves in the cistern, which is an Equilibrium valve, and when the shower is running the pressure is too low to keep it shut.

I like equilibrium valves and have never experienced it, but it might be the cause.

Take off the lid and photograph the filling valve. Also look to see if there is a service valve on the filling pipe, and guess how old it is, because you might need to change it. There are a few major makers.

How high above the cistern is the cold water tank?

Do you use a shower pump?

Thanks, PJ. I'll take a pic later. Getting the lid off the cistern is tricky, because it seems to be connected to the button somehow.

Yes, it's quite a new cistern, but has only started misbehaving recently. It's a loft bathroom so the cold water tank is not a lot higher than the cistern. Yes, it's a pumped shower (but the cistern sometimes piddles when I run the bath, which has a feeble flow). Tis all very odd.