Hi Mike,
Just noticed I never replied to this.
1. Assuming the photographer was not high in the Cairngorms, then I think there is enough in the photo to confirm
C. diversa.
2. Looks like the photo of
Physcia tenella on my website too.
Because it is.
The problem with Dobson's pic is that it shows only the asexual reproductive stage (soralia at the ends of the lobes) and it does not show the sexual reproductive stage, the apothecia (discs). That's OK, except that he doesn't mention them in the description either, and in the key to
Physcia he says "Rarely fertile" - which, with all due respect etc. etc. is complete and utter ... um, ... well, it's not true.
I can just make out soralia in your photograph, which
P. leptalea does not have. So yes, your photo is
P. tenella.
Yes,
P. leptalea is rare but I fear Dobson-users will be keying out fertile
P. tenella and
P. adscendens as that species rather often.
[In most respects it is, of course, an excellent book and essential for anyone trying to identify lichens]
Alan