In uk.d-i-y, Dave Jones wrote:

As you Joining 10 mm cable you really need to use a connection block of
greater than 65 Amps, i.e. same rating as the cable.

Really? I Think You'll Find that sizing according to the overcurrent
device - a 45A presumably-type-B MCB, in this case - is the relevant
design procedure. One could plausibly argue that the lower load current
of 40A (9.6kW/240V - yes, 240V not 230V since the 230 figure is a
regulatory fiction and should the supply drop to 230V in some distant
future the load drawn by the heater elements will also drop) for the
fixed-equipment shower, which has no way of producing an overload, could
be used as the circuit design current: but that's a bit of margin-shaving
in which the sensible person would not indulge. But sizing for a 65A
current throughout, when the 10mmsq cable has been selected to give a
decent margin for voltage-drop/earth-loop-impedance/passing-thru-thermal-
insulation or whatever, rather than the "might just do it on a good day
with the wind in the right direction" use of 6mmsq, is over-egging in
the other direction.

Stefek