So, should the oven be RCD-protected (leave things as they are) or not
(get
him to put things right)??

I reckon you'd get the fridge, freezer, oven and CH on the 20A radial, which
will run in 2.5mm cable.

Most ovens are 2.2kW plus a bit for the fan. Fridge and freezer won't run
continuously and are probably only about 300W each when firing. CH will take
very little. Even if the oven takes 3kW, there's still 1.6kW to run the
other appliances, which is easily enough.

Christian.