I received a helpful email from Killian Mullarney pointing out that the jaeger I had labelled as Parasitic here was in fact a 2nd calendar year Long-tailed.
He outlines a number of pointers to the correct identification:
1. If you compare the underwing pattern [with this bird] you will notice that the Arctic [Parasitic] has a typically large, white 'flash' on the undersides of the primaries, whereas the bird in question has a much more restricted flash, seemingly barely extending to the outermost primaries. The restricted white flash is, as I'm sure you're aware, a strong pointer to 2nd calendar-year Long-tailed, reflecting the fact that, unlike Arctic, adult Long-tails lack the flash entirely [in adult plumage].
2. The Long-tailed also has a much whiter and apparently unbarred rear underbody.
Here are some more pictures of the bird:
In retrospect, a more careful inspection of my pictures would also have revealed the following pointers toward Long-tailed Jaeger:
1. Contrast between pale grey-brown upperwing coverts and darker secondaries
2. White 'flash' on upperwing restricted to the feather shafts of the outer two or three primaries
3. Strongly barred uppertail coverts
4. Slim, graceful appearance
5. Pale-headed appearance
Still, I learnt more by making a mistake than I would have done by getting the id correct first time!
Incidentally, the PM500 Club now has its first 5 members. Scroll down to the bottom of the page if you are curious to know who!