But that will gain him absolutely nothing. You're replacing a ring
which requires a cable rated at 20A with a radial circuit also requiring
a cable rated at 20A, whilst the cable (as instlled) only has an effective
rating of 13.5A.

Sorry, I wasn't clear. It would become a 20A ring. A bit non-standard, I
know, but safer than running 32A.

with the fan heater off an FCU directly and the sockets off another
fused 13A spur. You can easily do this as you ran as a ring, so
you've already got two supplies to the sockets, provided the FCU
for the heater can be on the first or last socket of the run.

The heater socket or FCU needs to be on a run of its own. In fact with a
single FCU or single 13A socket, it could be an unfused spur.

Another misunderstanding. I'll try to explain better with some ASCII art.

OP's original plan:

+----SKT1-----SKT2------SKT3----SKT4-----+
| |
---+-// original ring cut here //-----------+-----

Proposed plan:

+----SKT1-----SKT2------SKT3 FCU------+
| |
--FCU---------------------------------------+-----
original ring left intact

I suppose the FCU on the right could just be an unfused spur socket for the
heater. The left hand FCU could be on the ring as shown, or installed
adjacent to SKT1. In either case, it does not require laying more cable.

However, I'm now getting confused as to the rating of 2.5mm cable layed
within insulation. Not having regs with me doesn't help. What is the
definitive current capability of it?

Christian.