Ok, so the tweeter protection bulb blew in my nemesis 1 x 15 cab.
I went to a car shop to get a replacement. The original is 24V 18W (no R276), but of course, they only stock 12V stuff. Apparently a 12V bulb will do the job, in fact it's said to make the tweeter sound less harsh, but it may blow more easily.
So I've used a 12V bulb for now but.............
If it blows, can anyone tell me where I can get an original?