Having second thoughts now, just in the process of removing the carpet from the C8 and there are another set of bolt holes nearer the doors in the floor, this means that the existing runners need to be removed and located to the outer bolt holes. 7 seater models have the seats nearer to the sliding doors and so the armrests will need to be removed also.
However, removing the seat runners is not easy as they are torx bolts and because they are exposed underneath means that they rust making them hard to extract.
As well as purchasing a middle seat (with the runners) you will also need the carpet from a 7 seater too!