Main switch or RCD, double pole for single phase supplies.

Any installation wired before the 17th will most prob not comply.  However, they have no need to provided that they complied with the regs in place at the time of installation.  You can recommend that they be up-dated (Code2 - an un-just coade 2 as there would be nothing wrong with it) but it should be a code 4.

Re '2 rcd split boards'.  I'll just say that for me, they do not comply with the 17th.  A fault on circuit 1 should not cause circuit 2 to fail!

To me it would be the same as putting a rcd to control a 2-way board?

3036 fuses will comply provided the circuit has been designed with them in mind.

(If I'm incorrect I shall stand in the rain for five days singing a Dylan song of your choice)
Rob