Just to follow up on this, I ended up doing a gearbox oil change (drain at sump, refill 2 litres). The car came with receipts for a gearbox rebuild about 40k ago, and from the look of the oil when it came out I'd say that's probably about right as the oil looked fairly fresh compared to what I've seen previously! In any case, put some new fluid in and it didn't change a thing, however about 4 days and 400 miles later I started the car and shifted into drive and it was nice and smooth, and I had 2nd gear back! I'm a little surprised to say the least, but I guess the fresh oil was enough to get things going again?
Many thanks again for your help Colin!