Thanks guys, unfortunately there’s no thread sticking out that I can see anyway, the thick washer is still there and seems to be holding firmly, if I pull it off, they may be enough of the remaining tube portion hanging down to get a decent grip on and hopefully unscrew it. I have bought a replacement nut ready to install. If that doesn’t work I’ll have to go to plan B which will be far more difficult.
It’s still not leaking so I’ll keep an eye on it and may leave for another day if this is the case, I’ll reset the tapes and tighten check the centre rear bolt and put some miles on it.