DavidM wrote:

[...] and the cable is buried in the wall for probably 2.5m of its
total length. Don't know until I get there what size fuse/mcb is
fitted in the CU. What is the maximum rating (kw) shower I can
connect to this size cable?

If clipped-direct conditions apply to the whole cable run (i.e. no
thermal insulation, no grouping with other cables, no conduit or
trunking) you can take the current rating to be 47 A, which means a
maximum of 10.8 kW at 230 V, supply and fusing permitting.

( Part pee.....yeah, I know!!)

It sounds as if you are doing a direct replacement, which is not
notifiable. It would of course be prudent to carry out insulation
resistance and Zs test on the circuit though, as well as testing any RCD
that may be fitted. Also check that the supplementary bonding is OK,
particularly if there's no RCD.

--
Andy