Using multi core cable instead of single core

No problem with using suitable stranded flex. Not a requirement but it may help to use ferrules.

Nothing stopping you using it if it is of adequate conductor CSA for the application in question.

Thanks very much both

Would I be right in assuming that the single core stuff is intended for fixed instalations where the routing will not often be distrubed, if ever, whereas the multi-strand (flex) is more for appliances which need to be moved around on a regular basis? That would seem to make sense to me. Flex tends to cost more than single core but the current handling of both (if sized correctly) is the same as will be the insulation if it's specified for mains voltages. So if the OP's got his flexible stuff for free, he's good to go I guess.

Power wiring used to be stranded, though not many strands. Ring mains used 7/.029, which is roughly equivalent to 2.5mm^2, with lighting normally 3/.029.

The general rule used to be if the cable doesn't move, use the cheaper solid core.

IIRC armoured cables are stranded...

In general, yes. For larger CSA cables even the fixed wiring cable is stranded (although fewer heavier strands) to keep it workable.

Is there a big difference for, say 10mm2 T&E?

I'm about to put in place the wiring for a shower and it's quite a tortuous route. I'd bought single strand as it's fixed, but could easily swap it over for stranded if it's going to be significantly easier to thread.

I confess to not seeing single strand 10mm2 cable, I've only seen cable of that size with what must be 7 strands!

Having to pull the cable through voids and spaces might become a little difficult!

Indeed! The electrician bent my arm a bit - strongly advised 10mm for future proofing. He didn't mention strands though - I thought one strand for fixed was the general rule.

it was a bit of a double take lifting the 25m reel - heavy! This is what I've got:

Much depends on what you are doing with it - the main difference is getting it into and out of enclosures and switches.

For a shower, the shower end and the CU end are usually not too bad, the ceiling switch can be a pig though :-)

(and the electrician was right - makes sense to use the larger cable - the cost difference is not that much and both will be a pain to wire. So you may as well endure it the once and have the option of pretty much any shower in the future.

Not kidding. I've replaced both the ceiling switch, and the shower in this house three times over the years. Wiring for the showers wasn't a problem. Those ceiling switches were a different matter altogether.

I did wonder why the electrician specified a wall mounted isolation switch . . .