SPD - cable selection

am I right in thinking it’s only there to provide short circuit protection rather than overload? 

The 2nd half of 534.4.10 would seem to agree!

   - Andy.

One hopes that there is almost no current in the MCB or the wiring to it, most of the time. The only times there is would be 

1) by design for some microseconds during a transient over-voltage when a few kA might flow and the SPD dissipates at peak perhaps hundreds of kW but only for a few microseconds,  and  thinking adiabatically, a wiring copper area of a few mm2 would probably do.

2) When the SPD finally dies, if it does so in a short circuit sort of way, when the large current will only flow at most for the time to operate the breaker, if the SPD does not fuse first, and to be honest a thinner cable would probably be OK then as well...
Mike
PS

In terms of responding to fast edges for lower inductance,  and best clamping a pair of flat palm braids would be better than twins of  round wire ,but such things are not compatible with normal wiring terminals and methods.

or 3). when a fault (short) occurs on the wiring between the MCB and SPD...

"or 3)."

Very true. A case where again, a thinner cable would probably suffice. There is no safe condition where 63 amps flows for very long at all, given the volume of the SPD itself and the absence of cooling air, water or rotating shafts to remove the energy.
Mike

"or 3)."

Very true. A case where again, a thinner cable would probably suffice. There is no safe condition where 63 amps flows for very long at all, given the volume of the SPD itself and the absence of cooling air, water or rotating shafts to remove the energy.
Mike