All,
Thank you very much, every one of you, for all your contributions and suggestions. (Especially the picture!)
I guess I should clarify a couple of points and then summarise my understanding of the advice:
First, the fitter (bullfitter), is intending to use an electrician. That's where he got his definitive advice from about the cabling that needs to be installed. Not so good for the major high street brand that thinks they're going to fit this kitchen (yes, I still might be able to change my mind).
Secondly, I guess I was slightly wedded to the 50A, because it was already installed and waiting. At the very least, we were engaged to be married.
Summarising my understanding:
Swap down to to a lower amperage breaker (not an issue, as they're not too expensive on the scale of things).
I CAN run a single cable from the CU to a SINGLE isolator switch that controls the two ovens. This can be 10mm2 (for future proofing)
This runs to a connection box the output of which feeds the two ovens via lower amperage cable.
There are other ways of doing it, but the one outlined above works for me.
I hope I have understood your comments correctly and thanks once again.
I don't wear a nightie.
