On Saturday, December 15, 2012 2:59:54 PM UTC, Caecilius wrote:
I've been experiencing occasional
nuisance RCD tripping for a few years now.

I presume there is no rubber cable in use - and this is a 1980+ installation? I presume you have checked any outdoor sockets & lights for water penetration, isolated any external shed circuit?

Hiring an RCD tester is going to be £80, perhaps £80+VAT, with delivery & collection. So replacing with RCBO is indeed the better solution and may pin the fault down to one circuit, well worth doing.

Buying an RCD tester can work - if you get an end of line electrical factor clone like Newey & Eyre or Alto Electrical "Own Brand" which whilst new can be a fraction of the Megger branded RCDT320 etc.