Thanks for all your replies. I think my best and safest course of
action will be:
1. Feed the cooker directly from the consumer unit to a 45A DP
switch (which could also have a combined socket to use for toaster for
example). There should be a spare connection in the CU as my son has
just had a combi boiler installed which means that the old immersion
heater has been disconnected and removed. I will check on my next
visit. I presume I will have to change the circuit breaker to 45A.
What size cable to run to the cooker switch?

A 45A breaker would need a 10mmsq cable; and TBH, cable costs are low
enough that you may as well use that. The next size down is 6mmsq, and
you don't want to be using that with anything bigger than a 32A breaker.

In practice, your cooker would run fine with a 32A breaker: the Regs
recognise that cookers don't run flat-out for long periods, so the
actual load your cooker presents over a period of minutes and upwards is
noticeably lower than the figure in its manual/on its rating plate. But
you should size the circuit so that the peak demand is safely met, and
when you're running a new circuit there's no good reason not to use
10mmsq, which (to a first approximation) means you can avoid worrying
about the route of the cable (both length and thermal insulation),
higher ambient temperatures, etc etc etc. It's also entirely acceptable
to wire in 10mmsq and fit the lower-rated (32A) breaker...

The necessary calculations and considerations to establish whether 6mmsq
would be OK are worthwhile for an electrical engineer specifying the
wiring of 200 identical Domestick Dwelling Units, where the shaving off
of a few quid on each house's build costs is the name of the game, but
not on a one-off d-i-y project. (One of the reasons why d-i-y often ends
up being done to a *higher* standard than "professional"/commercial work...)

2 Take a spur off the existing double socket via a FCU with
suitable fuse for the extractor hood. I don't think it is a separate
ring for the kitchen and he has a fridge/freezer, microwave, toaster
and kettle plugged in. There are also the usual other household items
e.g. TV & HiFi in other rooms on this downstairs ring, so I will check
the total loading. What factor of safety should be used when comparing
total load to the capacity of the ring?


None of the appliances you mention are all that high a load, and don't
run for long. One-ring-for-the-whole-ground-floor only starts to get
marginal if the kitchen's heavy on water-heating appliances (dishwash
AND washing machine AND tumble-dryer AND kettle), AND is close, in
length of cable, to one end of the ring. Under those conditions, the
bulk of the load is supplied by the shorter side of the ring (the
current splits according to the resistance of the two paths, which is
mroe or less directly related to the length of the two paths, though
there's a small self-limiting effect with the resistance of the shorter
leg increasing as it runs warmer). Practically, you/your son should be
fine with the one ring; if it's convenient for the cooker control panel
to include a socket for the kettle, that's one useful way of keeping the
d/s ring load well below the 32A rating. 32A allows for a *lot* of load
- 8 glorious kilowatts...

3 What is involved in completing a Building Control form? Is it
complex? Will it require a visit from someone to approve the work
before/after I go ahead?

What? who? building control? ;-) If you want reassurance, your (son's)
money will be much better spent on directly getting an electrician to do
a "periodic inspection report" to suss the state of the whole
installation, including your planned changes, than satisfying the letter
of Part Taking-the-Pee...