I'm a bit puzzled by a problem on my P6R.
I was on the way to work on Friday, and it suddenly started slipping in the small sprocket. Once it started, it did it consistently, several times on each pedal stroke. I had a quick look and didn't see anything, but found that I could put it on the large sprocket and still have three hub gears with no such problem, so did that for the rest of the day.
I've been adjusting the derailleur stops, using the Brompton instructions. It seems a lot better (one slip every ten or so pedal revolutions), but I can't clear the issue. I'm not entirely convinced that the root cause is in the derailleur adjustment, to be honest. For one thing, adjustment problems develop slowly - but this started quite abruptly and then was consistent. I even wondered about a broken sprocket (happened to me once on a Regina freewheel, decades ago), but it's pretty clear that it's not that. Watching carefully, it does seem that the large sprocket is a bit out of roundtrue - it moves from side to side a bit, like the ring on a cheap chainset.
What does the panel suggest? I've ruled out hub gear issues on the basis that it's correctly adjusted, and doesn't slip in the large sprocket, in any (hub) gear.
As an aside, I can't quite see how the Brompton derailleur works, in one key respect. On a conventional derailleur, you use cable tension to position the derailleur correctly, and the limit screws are just a backup to prevent the chain going off completely. On the Brompton, the over-ride spring means that cable tension is pretty-much outside your control. So, it's almost as though the chain itself keeps the derailleur correctly positioned, rather than the other way around. Of course, the cable and spring are limiting any side-to-side pull, but not, surely, setting it as precisely as in the conventional design?