Simple NCT test question

In order for the Prioity List system to work for us, do we need to have already booked some slot or other? Or do we maybe have to have nothing booked?

I'm also not sure what happens re payment(s)/possible refunds around all that, but there's no rush to clarify that. I expect the Priority list team will come up with some earlier appointment (and am sure the financial stuff will be fair).With a lot of other things on my mind, I overlooked my NCT expiry, which is this Thursday. I only realised last Tuesday, when got a motor tax reminder email, and happened to scroll down it. I immediately went to the booking page of my usual test centre (the only one I have the time to reach at all easily), found the first available slot was the 25th (March), bagged that, and requested a place in the Priority List (which was confirmed the next day). I thought I remembered payment for NCT being taken from our cards only when we've arrived at the centre, but last Friday's booking confirmation email for the 25th tells me to make my payment in advance via the link provided.

I find the official info a lot clearer than it used to be, but still manage to run up against little ambiguities! (And speaking of those, one thing I just will not do is risk driving uninsured, despite the difficulties of going anywhere from here without driving.