Thank you for all your helpful advice - it is much appreciated.
I have just looked up his Waterlow scale score and it is 17, so not a trivial risk level.
We have been keeping a close eye on it all day - it is no worse, and no significant pain. We have decided to see how it is overnight and talk to DN in the morning.
We do have the cavillon cream and the spray. We have only used the cream up till now, but used he spray today.
I am wondering whether he needs a better mattress and cushion. The cushion looks very hard to me; and he was still in pain last night even on the mattress. The problem is that, although he walks about a bit during the day, he cannot change position by himself during the night - apparently that is typical of PD. He has an NHS profiling bed but that does not help him to turn on his side, which is what he needs to do - I find it very hard to turn him on his side, even though he also has a slide sheet. The bed only changes his head position up and down, which does not deal with the problem.
I will talk to DN tomorrow. Thank you all for your advice - this is all new territory for me.