I’m going to swap out the manual gearbox on my D2, what other bits inside the bell housing should I replace while it’s out?
The clutch has already been done but what about all of the release arm gubbins? Are they likely to fail randomly?

Depending on exactly what was done when the clutch was done.
Personally if I was to go to the trouble of pulling the gearbox out for whatever reason, I would also replace everything I could see while I was inside the bellhousing, from the DMF flywheel, clutch assembly and friction plate, thrust bearing and release arm, especially should any uprated versions be available.
While the flywheel was off, I'd also consider changing the rear seal on the crankshaft and the first motion shaft spigot bearing too.
Gaining access to the inside of the bellhousing doesn't happen every day so I'd be tempted to bite the bullet and get everything rebuilt while the opportunity presents itself.

@brian47 is the clutch fork on a D2 not a cast steel part with a nylon pivot bush that would probably be worth replacing?

For sure the release bearing with an uprated HD one and spigot bearing.

I previously swapped the engine so rear oil seal and spigot bearing was done. Also 3 part clutch and DMF.

I was concerned about the fork etc so the nylon bush sounds like it should be done. Is there a small metal clip or retainer somewhere in there?

See this video. Can give some ideas... I have followed this when I have removed box mainly to change the rear oil seal.