6. Finally looking up the ratings for these cables in the IEE regs &
comparing with the TLC catalog, why the differences?

Email TLC and ask, could be a number of reasons.

XLPE probably (modern higher temp rating).
Think it was XLE that deteriorated over time for HV links & went boom.

2. Ducting: Instead of burying it in bare soil it looks more sensible
to run it inside a duct,

No need, but do lay a "cable buried below".
o Ducting is good at filling with water
o Ducting =/= run net/phone cable next to power cable
o Ducting = run net/phone cable in flex-conduit next to power cable

5. Cable size: nominally 6mmsq cable is adequate for the garage load
(ring + freezer radial + lights), but am wondering about using a
larger cable - eg 16mmsq.

Consider how long your ring-main loop can be serving a typical 50m^2
area (subject to voltage drop & earth disconnect time). Your proposed
run is only a few metres, and XPLE cable rating is higher than PVC FTE,
so 16mm^2 may be somewhat overkill unless you can on a shower :-)

Note the XLPE rating (which is re higher temp insulation IIRC) may be a
factor in your sizing of FTE to feed it from the CU or from it to other CU.

If you are using big enough machinery to really cause the lights to dim:
o Consider a non-maintained emergency light in the garage (£19-22)
---- whole-garage RCD = lights go out & excitement begins
o Consider a garage CU with RCBO for ring, plain MCB for lights/freezer
---- new MK/Hager/MG 1-module 32A RCBO is £19-26 are regular on Ebay
--
Dorothy Bradbury