What with labour and all the rest of it, a replacement gearbox would cost something like £3,000 -- which I hadn't anticipated spending just at the moment.
It's done 120,000 miles, and even if I gave it a new gearbox it's not guaranteed that I'd get another 60,000 miles out of it before something else substantial needed doing.
So it looks like I'll be getting another car in the next few days.
Times are hard and budgets are tight, so it seems likely that my next car will be quite a small one. This will have implications for things like the amounts of stuff I can take to conventions, how useful I am helping people to move stuff from point A to point B, how agile you'll need to be to get into and out of the back seat, and so on.
Ah well. I'm going to go to the dealership now, and ask them how little I can pay for a car and still have a reasonable expectation that I won't be crippled after four-hour drive.