Thanks for the replies.
I think I'm realising I'll actually only need one cabled switch. The new cylinder is still in the box & I hadn't read it -- but it looks like the "other" coil is for solar power, which I won't be using...
I was swapping out two immersion heater switches.
When I took the old switches off, I was surprised to see that one switch had 2 of each wires (E N L) connected at the load terminals. Eventually I figured out that the second set of cables was actually the supply going back into the wall &...