RCD Random Tripping Issues

I had it with my old house, I think likely was spikes on the supply, as using a RCD tester and an insulation tester never showed a fault, the only thing I did not have was a clamp on ammeter able to check leakage was under 9 mA, but I suspect it was under 9 mA as well over 1MΩ using a 500 volt insulation tester.

I however did not have the options of fitting RCBO's in that consumer unit/fuse box, since you do that does seem best option, and I would do as you suggested replace the RCD with an isolator, although @EFLImpudence is correct can just as easy connect the two bus bars together and use the old MCB's as blanks to fill the gaps. Or fit a Surge protection unit to fill gaps.

Since your not complaining about the RCBO working the upstairs lighting is seems likely not a spike as it would likely have also tripped the RCBO, you have not said what readings you have got when testing, the Hager ADA332G at around £23 each that means 9 will cost around £207, I got an insulation tester for around £35 new, although no calibration certificate although not really needed for what you want.

Not found cheap RCD testers.

Surely it would be cheaper and easier just to do a bit of testing to find out where the fault is?

Any out side lights or any external electrical fixings fish pound pump ect ?