Double bank dual RCD Board

[JohnW2]

Very interested to hear if anyone knows of a product that can be retrofitted into an existing Wylex CU.

We don't know what particular Wylex CU you have. Although I haven't found any single-pole ones (like the one flameport illustrated), there are 2-pole and 3-pole ones like this around (as I said, you can think of them as permanently-'on' main switches) ...

The product code for the double-pole one is NHDPDC and that for the triple-pole one is NHTPDC, but I can find no trace of a hypothetical "NHSPDC", so it may not exist as a buyable product.

I agree. Which type and size of cable should I use for this purpose?

Presumably the same as you would use to feed the second RCD in a single-row dual-RCD board - I would imagine most commonly 'tri-rated' 16mm².

Kind Regards, John