RE: Which charger?
It sounds like you will need to have a new run of cable from your consumer box to the garage, yes. Chargemaster would be unlikely to charge for this extra effort, it is after all just more cable, but from what I have heard BG will be more than happy to charge you.
I suggest you clarify this detail with BG before they turn up. You may find that they can already clarify the additional charges. I would suggest you say to them that you expect a 20m additional cable run to come from the house to the garage.
You could buy yourself 25m of armoured 6mm cable and bury it where you want it to run, and leave the two ends for the electrician to wire up. But I'd have the conversations first, and if BG want to charge you more for the extra cable then ask CM to compete. It is a CM unit that BG will fit in any case.