Thanks both.
I agree it is an odd fault! They've died in stages so I'm not sure whether it's the base station or not. The speaker-phone function doesn't work either so I think it's the actual loudspeaker on the handset that's duff........... I could try poking around in them for loose wires or something but I'd probably end up mutilating them beyond use!!!!
It's one of those bugging situations where they're just out of warranty, now been discontinued so can't buy replacement handsets individually, don't warrant repair as we live in the so-called 'disposable age' and yet they're too new to just hurl and replace with >£100 of new phones
Thanks for the comment regarding filtering - will bear in mind.