So I decided on a ring.. I do have a fairly large house, and
extending it through the conservatory has added an extra 20m
to this (10 meters each way), so I hope that this won't be a
problem!

It will, unfortunately. A 2.5mm cable buried in insulation is not suitable
for a 32A ring main. If you can't redo in thicker cable (i.e. the windows
have gone in), I'd run the new section back to the consumer unit and use a
20A MCB. That will be enough for your 3kW heater and a few other bits.
Alternatively, run it as 2 13A spurs, 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.

Christian.