So my car already has a service plan, with just one minor service left, which isn't due for 6 months. Apparently you cannot take out a new plan when there is already an existing one in place.
I've booked in for the free inspection, to make sure the car qualifies. If it does, I'll treat myself to an early minor service, then take out the plan. Fabulous, thanks OP for bringing this deal to my attention.