So, I will protect the sockets with an RCD until I get the rewire done.
Currently there is a 32A MCB for the sockets. Do I simply need to
replace this MCB with an appropriate RCD or is the RCD in addition to
the MCB?

If it is an RCD, then it is in addition to the MCB. You could put it in a
separate box after the MCB to just protect the socket circuit.

However, you can get an RCD that replaces an MCB. It is called an RCBO.
However, you may find it difficult to find a suitable device for some older
boards. If you have a DIN rail system, then you should be able to find a
suitable single width RCBO. These are a direct replacement.

Note, that when you get the circuit sorted, you should probably install an
extra few circuits in the kitchen. It is usual these days to have a separate
kitchen ring that is RCD protected and to have one or more circuits for
fixed appliances (washing machines/freezers/fridges/tumble
dryers/dishwashers) that is not RCD protected, so that your frozen sausages
are still frozen when you get back from holiday.

Christian.