George, It's good to know I'm not alone with this fault!

I'm hoping that if we've both got this problem then it could be a
common fault with this chassis with a known resolution. Oh well, here's
hoping that someone will come along with a "Check X,Y & Z" type fix for
us.

It's been over 15yrs since I repaired brown goods fulltime so I'm quite
out of touch. That said, I'll try and put some time into this over the
weekend and make some progress.