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Thanks. I would only have swapped like for like if the RCD was faulty. I (carefully, and having read a lot about consumer units first) swapped the two identical RCDs round and we still have exactly the same problem. So it's not the RCD.
I wasn't overwhelmed by the job the sparky had done who fitted it to be honest: one of the MCBs is missing the plastic clip to keep it on the DIN rail, no gromets used where the cables come in, a few cables that could have been installed more tightly and the plastic cover for the busbar wasn't installed. And hadn't cleaned up after himself inside the CU. But none of these are the cause of the issue.
I'd agree that telling people on the internet it's fine go ahead and take your CU apart is a bad idea...
Fox
Just as a bit of added info. One reason consumers are not encouraged to replace breakers or 'allowed' to create new circuits is the breaker is chosen based on the type of cable, where the cable is located and the overall resistance of the circuit. It's a relatively complicated process to calculate and test although it generally leads to the same configuration in domestic installations.
If a consumer changes a breaker for a different rating there is a chance that the circuit won't be adequately protected in the case of a fault. So fire or injury could be the result.