Matt wrote:

Which is fine right until the point the oven element fails and the
break in the earth conductor in the consumer unit that has gone
un-noticed for months/years/forever means the fuse doesn't blow and
the oven casing rises to mains potential.

That displays a fundamental misunderstanding about what the 30 mA RCD in
a split load CU (in a TN earthed installation) is for. It's not there
as a form of backup protection in case the earthing is dodgy. Its
primary purpose is to provide supplementary protection against direct
contact with live conductors, particularly outdoors - the cut hedge
trimmer flex scenario, if you like.

The On-Site Guide is quite unequivocal about which circuits should be on
the RCD side of the board:

"30 mA RCDs installed to provide protection to socket outlets likely to
feed portable equipment outdoors should protect only those sockets, see
Fig 3b."

The guide goes on to state that RCDs installed for indirect contact
protection (where the earth fault loop impedance is too high to allow an
OPD to perform this role) should have a rated tripping current of 100 mA
or more.

Put the cooker on the RCD!

.... is the wrong answer!

--
Andy