Three Q's from me today!!

This one is odd, first;

In this house we have an issue where if you turn tap on, flush the loo, anything that involved water flow, the shower (electric one - one of three showers we have) water goes cold, causes the mrs to have a dicky fit. Other shower is normal hot water tank pumped, seems unaffected, other other shower is a daft cold water only one which is stupid.

Anyway, woman came to check water meter reading and said there's some water usage, despite all water-using stuff being quiescent, get it checked. Never bothered.

Lately, noticed a water-tank-filling-up noise ever so often, causing said mrs in said shower to get said well-annoyed accusing all and sundry of turning a tap on or flushing a loo, against all instructions.

And I can hear it doing it.

However today I checked water meter - stationary, no water being used, checked all loos, taps, garden pipe, all fine.

But even so often I hear that filling up noise!!!

Ideas?