This piece has been discussed on this forum long time ago, but I can't seem to find the relevant threads.

I would like to know how piano teachers go about teaching the very tricky bits in the first reprise, where the top note of a tone cluster is trilled.

I can think of an easy way out:
When the LH has a quaver rest, it can take some of the notes in the cluster, thereby allowing the trill to be played with fingers 3 and 4 in the RH.

However, this cannot be done when LH has a chord sounding on the downbeat. The RH must play the trill with fingers 4 and 5 and letting the other fingers play the rest of the cluster. This is extremely difficult for an average Grade 8 pupil.

Yes - this sonata is more difficult than those in at least one of the pairs on the DipABRSM syllabus (K208/K209)

Well you could do that ...

... but seeing as you have to play some of them with 4-5 anyway, why bother

True. It is not that easy for this LRSM student either. But the reason things like this are harder than they should be is that most students leave it way too late to start practicing this and other so-called "advanced" skills. It should have been introduced gently at Grade 5, so by Grade 8 trills and other ornaments with 4 and 5 would be no trouble. The same is true of things like polyrhthms. It is not so good waiting till you have to playthings like 3 v 4 and 4 v 5 in a Grade 8 piece before you start working on them. But if you arrive at Grade 8 preparation lacking a skill you should already have learned then you just have to knuckle down, practice it every day, and train yourself to do it. It is all part of becoming a pianist.

I agree that by Grade 8, one should be able to trill comfortably with fingers 4 and 5. But, in this Scarlatti Sonata, the player is expected to do a lot more than that. What I find very difficult is the quick release of the two non-chord tones in the cluster while keeping the remaining notes in the cluster and playing the trill with 4 and 5.

I feel like even someone with an FRSM would have to practise a lot to get this right!

I am not disagreeing with you! As I said:

and

Perhaps the AB just got this one wrong. One of last year's Grade 8 Bach P&Fs (Bk 1 No 17) was harder than some of those on the Diploma list

I did that P&F for grade 8 a few weeks ago (and flatter myself i could play it well) and have now started on K259 & K260 from the diploma list. I could hardly believe how much easier it is to play the notes of these pieces by comparison but getting it right musically is another matter!