The incident posted at the top of this thread happened only days before we went there. We were mindful of it when we took our RRs on the beach and didn't have any problems with the cars. A local shopkeeper told us that regularly a glint can be seen at various places on the beach at low tide and it is the roofs of cars that have been lost to the mud and quicksand.
RRB's dogs were fine, but ours - we don't know whether she burned her paws on the sand or whether she had an allergic reaction to something in it, but the skin between her pads was red raw and she ended up at the local vet's (on the main road in Burnham in case anyone needs it) with some rather nasty ointment. She was fine after a couple of days. We then took her on the beach at Weston-s-Mare with no recurrence of the irritation.
