Chris wrote:

Ok. We may end up putting washing machine/dryer in there at some point,
as well as an electric heater, so I'll have to work out how much load
that all comes to.

That might total more than 20 amps but are you likely to have both on
at the same time? Assuming 3kw (12 amps) for the washing machine and
dryer then a 1kw fire would be fine, a 2kW one would be marginal.

That would certainly simplify things a bit.

If I need to up the load in the workshop, I'd then presumably need to
do something like wiring it as a ring starting and ending at the garage
CU.

Assuming the feed to that point is OK, yes. In fact since there's a
30 amp fuse in the house protecting the feed the garage CU is possibly
redundanat though it's probably worth keeping it. I think (as I said
before) I'd get rid of the workshop CU and keep the garage one with
the MCBs as they will give you discrimination with the feed being from
a 30 amp fuse in the house.

--
Chris Green )