Scott wrote:

I'm wondering if you need an MCB with a different swiching characteristic -
type C or D. Basically you get a slight short circuit on switch on which
could be knocking out the MCB, a C/D type will allow for this before
tripping.

Absolutely not. Storage heaters have nichrome elements which do not
have any significant switch-on surge. The small amount of inductance in
the circuit will not have any significant effect either. A type B
breaker is entirely appropriate here. In any case changing an existing
type B MCB to a 'C' or especially to a 'D' without checking the earth
fault loop impedance etc. could be very dangerous.

As Lurch has said this is a mad way to wire up storage heaters, but that
doesn't explain why the MCB is tripping. The small overload when all
the heaters are on (~36 A through a 32 A device) should not be enough to
cause the MCB to trip. So either the MCB is wrongly marked or faulty
(fairly unlikely) or something is drawing more current than it should
be. The OP needs to get hold of a clamp ammeter and start measuring
currents.

--
Andy