Well, at the weekend, I finally got around to removing the gearbox, stripping it and rebuilding with spare parts from other ribbed-case 'boxes I have.
The worst malady was a missing tooth from the 1st gear cog, which caused a clanking noise in 1st and reverse. Shame, as the cog was otherwise in perfect condition.
I also noticed that the 1st/2nd gear selector fork was very worn where it had evidently been rubbing against the cog, which I guess had led to the situation where it had jammed in reverse and 4th, with 1st and 2nd nowhere to be found.
Anyway, I replaced the 1st gear cog for one with no missing teeth, and as a precaution, fitted a replacement 1st/2nd gear selector fork with a loose-fitting washer from another 'box, which should hopefully prevent the dreaded scenario of the detent balls in the 1st gear synchro hub jamming from the 1st gear cog being pushed along the hub too far, allowing the balls to pop up, as is quite common I gather.
Got it all back together yesterday evening, and it bench tests as OK, so I'll hopefully fit it back in the car soon...