NE-ELEC-12 said:

We have concerns regarding the use of a cable with multiple single phase circuits within it

Doesn't that happen if you have TPN and split it at the load end into 3 no. SPN circuits?

In addition, it happens with singles in conduit or trunking?

It does raise the question as to how the current-carrying capacity was determined, because BS 7671 (and probably manufacturer's data) doesn't cover this use-case for multicore cables (it does for singles in containment, using grouping factors, but there's no evidence that multiple conductors in the same cable can be grouped in the same way using the same factors).

NE-ELEC-12 said:

nd the use of a common CPC for both circuits, is this allowable with in the regulations?

Whilst BS 7671 requires a cpc to be provided at each point in wiring, and, where ADS is used, to be run in the same wiring system (or its immediate vicinity), there is no specific requirement that a cpc is required for each individual circuit. Best analogy is singles of multiple circuits sharing metallic containment where the containment is the cpc.

If open-PEN disconnection devices are used on each of the circuits separately, and are upstream of this cable, the protection of the OPDD is invalidated. See Regulation 722.411.4.1

If I understand correctly there are 2 neutrals in the cable, as well as two lives ? That is OK, and the CPC will sensibly only be called upon to serve and disconnect a dead short fault in one charger at a time, so that sounds OK too.  It is a slightly unusual use of  SWA intended for 3 phase, but not breaking any rules,  and fine so long as neutrals are clearly identifiable. 

It sounds like you could add a 3rd charger if you put a 3 phase ganged breaker at the supply end and then had a little breakout pillar beside or near the chargers to break out the 3 phases.

Mike.

As has been said, nothing wrong with two single phase circuits in the same 5 core cable. Also nothing wrong with these two circuits sharing a common earth.

However in my view, the cable should be de-rated to about 85% of the published rating. I presume that the published rating assumes a 3 phase, 4 wire circuit in which only 3 cores are fully loaded. Use for two single phase circuits will result in FOUR cores being fully loaded which will produce more heat than three fully loaded cores. In many cases the cable size will be the same, but the next size up may be needed if the thee phase rating was already only just sufficient.