Just wanted to check that I was reading the OSG correctly!
If I use Reference method A (i.e. enclosed in conduit in insulated wall) then 2.5mm flat T+E cable will be sufficient for a ring circuit.
Presumably I cannot put 2 cables in the conduit (i.e. to and from the socket) as this will derate the circuit under 20A and I would need to increase the size of the cable.
The alternative is to ensure that the cable is touching the inner surface of the plasterboard and that the two cables do not run in close proximity to each other.
Cheers
Martin