Given the hassle it'll be to get him to put his mistake right, should I
just
leave the oven on the protected circuit?

How many of the following devices are there (or expected to be) on the RCD
side:

1. Washing machine.
2. Tumble Dryer
3. Dishwasher
4. Microwave.
5. Kettle.
6. Oven.

Christian.