Ok TE post dump. Doing some catching up here after a bit of a hiatus...
So the Solartron 7150 plus is first candidate. Was stripped down and all the capacitors checked. They were all good! Winner.
Then the mains filter was swapped out carefully with the proper replacement part. I can't bear to see some of the cheap hacks people have done which involve cutting chunks out of things to save a few quid. The new one is almost identical other than it takes 20mm fuses instead of the long ones. I have just inserted the old fuse holders into the new unit as I didn't have any 250mA slow blow 20mm units. It comes with the filter pre-wired to the dual fuse compartment. I pulled off the spades and slipped over some heatshrink to avoid issues with accidental contact. The original one had a 100k carbon composition resistor across the mains as well to discharge the X2 capacitor in the filter. I omitted that as it was too much faff.
After this it was powered up and checked and all was good. The old mains filter was discarded.
Next came the LED upgrade. This is a bit of a pain in the arse if I'm honest. I snipped the old LEDs out, removed the LCD display and the diffuser panel, cleaned out the PCB holes properly and reassembled with new white LEDs. The unit was fully reassembled and powered up. All good
It was compared to all DMMs I have and the 8060A, 25 and Solartron agree with each other. The 87 is however slightly off (7 counts) on DC volts so I need to look into that.
I will clean the whole thing up properly another day.