Pete Verdon wrote:

[...]
The obvious approach is to wire up the sockets (probably three doubles
right next to each other) as a radial, and just connect it into the
"downstairs ring" MCB. Logically this seems like a spur off the ring,
but is it likely to raise any eyebrows?

Like that it would because an unfused spur from a ring should only feed
one single or double socket, or one item of fixed equipment via an FCU.
A fused spur will be fine for your application though - spur off from
the origin of the ring in the consumer unit to a switched fused
connection unit (with neon, if you wish) fitted with, say, a 5 A fuse.
This can then feed as many sockets as you need, which you can label "for
IT equipment only, load 5 A max." or similar. The switch on the FCU
provides a convenient master isolator for all the equipment.

The only other source of power in the cupboard is the FCU for the
boiler. This would presumably be on the ring since there's no separate
breaker for it, so I guess I could take a spur off here instead.

Don't spur from an existing spur, for the reason given above. There's
no objection to two spurs being taken from the same point (including the
origin) though, subject to wiring space and terminal capacity.

--
Andy