Wow, catching up on all that was interesting
Quick update: All MCB's have now been replaced with RCBO's and I've had no issues since. Granted, I've had the cooker replaced since then but it's very unlikely that was causing the issue.
In response to the "why buy an earth leakage tester?" question. I'm just a very technical person with an interest in almost everything. I wanted to narrow down the problem myself. I'm not saying I'm qualified to do what most people do on these forums, but I've learned some things and that's better than when I originally posted the question. Totally unrelated, there's a cheap earth leakage tester on eBay if anyone is interested