Sorted!
New gearbox arrived today and it's now bolted in and appears to be working fine. There's a few little bits left to do tomorrow - topping up with oil, reconnecting the starter motor, battery etc. but it shouldn't take much more than an hour or so. The breakers I bought the gearbox from had originally sent a 1.3 box by mistake but they sent a replacement for free and let me keep the first one. It's the first major work I've done myself and I've learnt a few good lessons:
1- I'm never going to a garage again except for tyres and MOTs. I had the front brakes replaced a while back and they charged me £110 labour. After taking the calipers and discs off myself to get the driveshafts out I realise what a rip-off that is.
2- The money I saved far outweighs the money I had to spend on tools. Plus, I now have some shiny new tools. I reckon I spent less than £300 in total (about half what it would have cost me if I'd taken it to a garage) and that includes gearbox, gear oil, tools and parts for servicing (engine oil, oil filter, air filters, fuel filter, spark plugs, coolant) which I am also going to do myself.
3- In future, I should remember to drain the gearbox oil before pulling the driveshafts out.
4- Cheap tools break. It's worth paying a little extra for decent quality gear.
Cheers all