Migue wrote:

I have upgraded the existing electric feed for the shower to 10mm twin
and earth because the shower I have bought is 10.8KW, and I will be
putting in an rcd in the electric cupboard.

I was also planning on hooking up the jacuzzi to this feed too. Due to
the rather large capacity of 10mm cable I was thinking of using an RCD
as low as maybe 5 amps?, which I was probably going to put in under
the bath itself (The rating of the jacuzzi is 4 amps).

It would be more conventional to feed the Jacuzzi from a nearby ring
circuit (socket circuit) via a fused connection unit. Since this is
notifiable work which you'll need to have inspected and tested, using
unconventional arrangements might lead to difficulty.

Make absolutely sure you get the supplementary bonding done correctly,
otherwise your work is likely to be failed.

Under the bath is not a good place for controls and switchgear which are
supposed to be readily accessible. Such items need to be in zone 3, or
beyond the zones or outside the bathroom.

At the electrical cupboard, assuming the voltage is 240 (or 230?) Thats
around 45 amps and with the jacuzzi thats around 49 amps , so i am
assuming I will have to use an rcd larger than 45 amps. I could make a
point of never using the two together but would prob rather it didn't
cut out if I did!

Your 10.8 kW (at 240 V) shower needs a 45 A Type B _MCB_ (or a suitable
fuse). There's no need for RCD protection unless the
earth-fault-loop-impedance is too high, e.g. if you're on a TT-earthed
installation.

Any advice greatly appreciated

If you're not sure of the difference between an MCB and an RCD, and if
you really think that a 45 A MCB would trip when carrying 49 A, then you
might want to think about getting an electrician in.

--
Andy