I rent a small flat in UK. The cooker ring main and the 13amp socket ring main are both protected by MCDs. The landlord says under the new regulations he is obliged to change to a RCD unit otherwise he will not be able to get insurance Is he correct in saying this is mandatory?

If this is an older home and has not had any RCD's installed, then I am sure the landlord is correct. Kitchens, along with bathrooms, and other locations must have RCD protection, along with the standard overcurrent and short circuit protection provided by the MCD.

Why are you questioning the landlord who is trying to keep you safe by adding additional protection?