Finally(!) got hold of an NG3 box and associated parts on Saturday. Drove to Cromer to get it on Saturday.
I told the breaker guy (small scale, works from his house) that if he gets any more I'll have the gearboxes, or I know people who will. I paid £175 including the clutch, flywheel, starter, linkage, etc. I know I won't use some of those bits, but it's useful to have them for fitting and measuring up. I think the next ones might be a bit more, as he's twigged that they're sought after now.
However, even if he wants £250 for one in future, that still seems like good value to me.