Either option is fine, but why the need for a new 32A circuit?1. I'll be creating a new ring main for the loft sockets and running these off the spare 32A breaker on the consumer unit. Can I include the two new "downstairs" sockets on this same ring main or should they be installed as spurs from existing downstairs sockets. It would be easiest to put them on the new "upstairs" ring that the loft sockets will be on, but is that against the regs and/or a bad idea?
What loads will be used on this new circuit - 32A is over 7kW.
Again either option, surface clipped is fine provided there is no risk of the cables being damaged by objects etc.2. For the wiring in the loft, is it a requirement (and/or a good idea) to use conduit/trunking or is it ok to simply clip 2.5mm T&E cable to the walls and roof timbers?
If the loft is like most, and will be stuffed with junk, then conduit or trunking would be desirable.
Round conduit would be my choice, but trunking could be used. 20mm will be fine for most applications, unless you have an excessive number of wires/cables.3. If mechanical protection is needed, would trunking or conduit be the preferred choice? Is one going to be easier than the other in terms of threading cables through, going round corners, t-eeing off to downstairs sockets etc?
25mm conduit will also require larger socket boxes, as the standard ones are not deep enough.
The PVC boxes are designed for use with conduit, as they already have 20mm knockout holes in them, and if not, additional holes can easily be drilled where required.
The 'normal' ones are useless for conduit, as they are far too brittle and will shatter into pieces if you attempt to drill holes in them. Even if you somehow managed to get a round 20mm hole in one, you will then find the plastic is too thick to accept the conduit adaptor.