4mm flat twin & earth if 1.5mm CPC also imposes EFLI limits.
Type-B CPD 32A is 1.20ohm max EFLI, 40A is 0.96ohm. So
as well as current limitations your house supply type (Ze)
combined with 23m in 4mm (R1+R2) may push EFLI over spec.

Cable technically needs derating for the run in trunking (which
is installation method 3 in the UK or about 12.5%). Some may
not bother if the run is very short. I assume the cable is not
*surrounded* in insulation anywhere (which if 500mm needs
a derating of 50% so your 4mm would be quite unsuitable).

Spark may have applied diversity calculations to the loading.
So just query him on the calculations and actual design, he
should be happy to provide as it justifies his skills/charge.