Hiya
Thanks again to all for your help.
I found a brand-new genuine Ford slave on Ebay. Which was great. It then took aaaaaaaaaaaaaaaaaaages to arrive (I'm in the UK and the seller was in the US) - that wasn't so great. However, once it finally arrived, I got it fitted and bolted the box back on. It was quicker the 2nd time around, so that at least was something. And I found out that it is possible to drop and re-fit the box with the transfer case still attached. However, I bought a purpose-made trans jack - I wouldn't try it if I were trying to balance it on a trolley jack single-handed.
Also, to add further fuel to the debate, I didn't need to remove the exhaust y-pipe. Which is just as well, as the nuts on it weren't shifting for anyone.
Having topped up the fluid and bled the system, I gave the pedal a nervous shove, and... it didn't leak!! Hoo-bl00dy-ray! Incidentally, was the placing of the slave bleed point intentional, so that you can press the pedal by hand, allowing you to see what's happening with the bleed pipe? Or just happy coincidence?
So it is back on the road, and I am well pleased. Now to replace the alternator, buy and fit a roll bar, get some wheels that actually hold pressure, replace all the window rubbers, etc etc etc.
p.s. that quick release clip that retains the slave pipe is certainly not quick to re-fit.