I took Dave’s advice and cleaned up the old gaskets (with the ever faithful Mr Muscle wipes 
) and the mating faces on the box. The received wisdom appears to be that there’s only two types of gasket sealant worth using on gearboxes; either a) “that thick sticky black shit” or b) “anything with RTV in the name“. After trawling the local motorfactors, Halfords, B&Q and Wickes I was starting to give up hope, but stopped in at the village garage on the way home and they had two different types of RTV sealant to choose from. 
I went with clear because it was the “Special Edition” and had a picture of a gearbox on it 
, but I could have had black too – both for less than four quid.
Apparently RTV stands for Room Temperature Vulcanisation, which is the important bit … the sealant cures at room temperature, rather than with heat, so if you let it set for 24 hours it should be a lasting seal, rather than using a sealant that’s trying to cure whilst the hot oil is still pissing out through it. That’s the theory at least …
I got all arty doing lines of sealant around the top of the gearbox casing, but must have been intoxicated by the heady mix of acids and chemicals in the stuff, as I didn’t think to take any pictures until after I’d put everything back together. 
Similar story for the box casing to bellhousing joint – the smell of this stuff reminds me of something from childhood, but I can’t quite place it. 
The fifth gear locking plate was hardly screwed down at all, so with no gasket and no sealant it’s hardly surprising it was leaking. Oh well, more artiness ensued. 
As I had access to it, I also popped out the bung on the other side of the tailshaft from the speedo drive gear and ran a line of sealant round that too. In for a penny …
Left everything overnight to cure, then “popped” the box back on, topped up the oil, refitted the propshaft and gave it a quick test while everything was up in the air. No sign of any leaks
I suggested to Selina that we pop out for a little test drive – like we had last weekend to the pub down the road – so she stuck a pin in the map and somehow chose the Malham National Park Centre in Yorkshire … a mere hundred mile or so round trip. 
There and back everything seemed fine, apart from some light scraping/grinding noises when in gear but off the throttle and a slight, but noticeable, clunk though the drivetrain when engaging gears. The scraping could be an old noise that I’m now able to hear because the box is quiter, or it could be the new box itself. I’ll see if it setles down, as it’s not been used for years. The clunking is like there’s a little play somehwere, but all the joints seem tight and the mounting points are solid too. 
Only other thing of note is that the box seems “tight”, almost like you can feel the inherent resistance in the box. I can only compare it to the old box, of course, which could have just been “loose” and this new one’s actually “normal”. 
I do know that if you’re in third and coasting through traffic in town, it’s much more inclined to try an bunny-hop as the revs start to drop off compared with the old one. Could be the box, could be the ratios, could all be in my head – guess we’ll just keep an eye on it.