[David Simpson]

GJ & Guys, its generally assumed that Electricity Supply Companies just provide pure 50Hz sinusoidal AC to properties. That's not the case these days. Ultra modern digital metering can be read remotely using data transmissions to & from the meter, via the supply mains cabling. For some years, Total Control heating tariffs use a timeswitch control signal superimposed on Radio 4's 198KHz signal. Some scallywags use phase changing methods to reverse the older analogue(rotating disc) meters. Some sub-stations have an amazing array of electronic & radio equipment, not just an old transformer & a couple of oil switches.
Any of this could be going on in your neighbourhood, and resulting in spurious small DC readings in your property.

Regards, David