Andrew Gabriel wrote:

The reason is most likely that the fusebox is only rated 60A in
total, and you may find your main fuse is also 60A in that case.
Sounds like you need to check if your existing supply can
support the additional 9.5kW load you are intending to add.
You may need the supply (or at least the main fuse) upgrading
to 100A and a new fusebox.

You're right, the main fuse is a 60A one. The cooker is gas though, and
the water is heated by a combi, so there's not that much demand for
electricity. It's a three bedroom terrace. The shower is for occasional
use only, predominantly in case the combi breaks down.

Can upgrading the supply be as simple as getting the main fuse changed?
Presumably the cable from the meter to the CU has to be heavy enough to
take 100A. I've no idea what size the current cable is, except that it
looks bloody thick! The only problem with changing that is that it the
fusebox is boxed in in a place that makes access to the incoming cable
very difficult.