Hi All -
@naylorpd was
mad kind enough to let me in for a look at the problem.
On arrival, clamping the tails did reveal a standing leakage of 70mA so there was likely to be a problem to solve. It turns out a fried USB outlet and a kitchen RFC fault were the major contributors. Fixing these two has reduced the installation leakage to 10mA with all appliances and boiler running etc.
The installation is 1980 (ish) TNCS PME , controlled by the original Wylex board with push button 5,15 and 30A mcb. There was a new kitchen fitted 10 years ago and one of the wall units has a mounting point in the middle of the cable safe zone. And sure enough probing the bracket through the back of the cupboard showed connection to the ring. Due to domestic practicalities we decided to deal with Kev's Revenge by disconnecting the damaged section for now. The RFC is now 2 radials with 15A OCPB, until a more convenient time to dismantle the kitchen, dig out the cable and repair it.