Pees all set. When do poops come?

[stopgap]

DS1 is 2.8. We started potty training three weeks ago, when he said he didn't want to wear nappies any longer, and from there he has done really well, telling me when he needs to go (about every 40 minutes, it seems) and often going by himself to the bathroom to use the potty. So we've only had one pee accident. Great.

He still takes a two- or three-hour nap in the afternoons, during which he will poop in his nappy.

Is this something therefore that he will only outgrow once he drops the nap, or is it something I can (somehow!) tweak?

Very common, boys can still poo in nappies right up to school age.
Just change him in the bathroom and let him see you flush the poop down the toilet for now.

We sorted out poops first after seeing the struggle sil had with Dnephew. DS clean day and night before 2 and dry at 2.9. Still had nighttime nappies but rarely needed them. Just easier than changing bed in the middle of the night.
Poops are easy if you have a child who has regular bowel. Also put potty in the loo or bathroom. It makes no sense to put them on the potty in any other room unless of course the rest of the family use a potty in the living room. They learn most behaviour by copying.

I know that he poops at around 4pm, but I'm loathe to wake him from his nap, as he's rotten when it's interrupted. But might that be the only way?

He used to have a poop every morning, too, but that's lessening over time.

I shall definitely flush rather than bag up and bin!

I know that he poops at around 4pm, but I'm loathe to wake him from his nap, as he's rotten when it's interrupted. But might that be the only way?

Give him the option of pooing before you put him down for a nap, but don't push the issue "do you need to poo, want to try on the toilet / potty?" is enough gentle encouragement for now.

You could also try moving the nap to an hour or so later.