Having got over my umbrage at being called boastful

, I've done a bit more research and the information published in various places is contradictory insome respects, though nowhere that I've found does it say 3 hours is only free for Morrisons customers. If anyone knows the definitive policy, I would be grateful to hear it.
Below the signs that I included earlier, there are also some yellow signs. I only had a partial image of that sign, but have since found a pic in a post on the Devils website last April, when the rink car park was being closed to enable construction of the new events car park.
https://www.cardiffdevils.com/news/parking-update-arena-users/
These yellow signs say categorically that parking is for maximum of 3 hours, with no return within 2 hours. It also says there is number plate recognition (NPR) in operation and if you overstay 3 hours, there is a £100 fine. This implies there is no need to, or way of, paying - you just have to be out within 3 hours. To my mind these signs have been superseded and should have been removed. Interestingly, they are under a Euro Car Parks header, the same as the other signs and the name that appears on my card statement whichever payment app is used. I would also say the NPR aspect is not true, because if it actually worked that way an awful lot of people would have been fined by now.
The Morrisons website simply says there is parking for up to 3 hours - i.e. in line with the yellow signs and not a reflection of what seems to be the current policy however it is interpreted.
As I said before, I cannot see anything that says you can only park for 3 hours for free if using Morrisons, but if you are concerned, play safe and pay from the time you park up.