Single RCD Consumer unit - 17th edition?

No, single RCD setups like that went out shortly after 15th edition.

Dual RCDs is probably the minimum. Depending on the number and type of circuits more (or RCBOs) may be preferable.

My unit was changed about 15 years ago and has 2 RCDs: 100mA incoming split to 30mA for the usual circuits. If a socket trips the 30mA one, the lights stay on. I don't know which issue of the Regs. were current then, but I'd mentioned to the Council about my Father not being able to move around in darkness so that might have been the reason.

That sounds more like a typical setup for a TT installation. Does he have power supplied by overhead wires and/or is he out in the sticks a bit?

The more commonly encounter comparable installation for non TT installs would have been a similar arrangement but with the 100mA RCD replaced with a simple switch.

I am the OP and have a TT installation. 1960's house in a road with mostly 100 plus year old houses and a 15th century pub (old coaching inn).

A split CU with 2x RCDs would be a good option then. Same brand so you can move the MCBs over. Whole house on one often works, but does risk total loss of power.

NT

Thanks. Even just the single split will mean not all the lights will go out and we can use lights plugged into the power circuit on the floor without lighting. Also means we can run an extension lead to the fridge and freezer if necessary. Perhaps the price of mcbos will nosedive eventually.