Thanks, everyone, for the advice.
Exhaust passed the MOT OK, so I must have done something right! The car failed on some other items, including the wipers

but that's just sod's law (thankfully the garage didn't charge for fitting the wipers -- it
wasn't a main dealer!

-- as it would have been pretty ironic that I could replace the exhaust system and had to pay someone to replace the wiper blades!)
Fitting: Once I had done the tab-bending trick and got the middle pipe on, it went together pretty quickly. The front joint was OK first time - I was worried that the old pipe might not seal properly, but it wasn't an issue. The back joint, between two new pipes, leaked from the top, and I couldn't see a way of pulling it together better without putting strain on something. So, I took it apart and slapped some GunGum on the top faces of the joint, bolted it back together, and pressed some more gum into the top of the joint, just to be on the safe side. I let the engine idle up to temperature, and then drove it across town and back. Still no leaks, so the gum obviously did the trick!
So, what have I learned?
1) PlusGas (or similar) makes it easier to remove rusty nuts
2) Rubber hangers are easier to fit if:
- you use washing up liquid as a lubricant
- you bend the tabs on the exhaust hangers until they are the same shape as the hole (or smaller) and then bend 'em back once the bits are fitted
- you can unbolt the hangers from the chassis first (may be possible for back two for 110 - make sure you refit them exactly as before to maintain alignment)
3) No gaskets are needed for the joints between front and middle, and middle and rear -- these are flanged joints and if you're lucky they'll be gas-tight without sealant.
4) If a joint doesn't seal first time, slap on some FireGum (or GunGum, if that's what you have!) to the mating faces and re-assemble.
5) GunGum doesn't have a loooong shelf life -- my still-sealed tube was 3 years old, and I had to rip it apart to dig some out that was squidgy enough to use:doh

6) Members of LandyZone are friendly and helpful!:clap2::clap2::clap2:
7) Fitting an exhaust requires a lot of muscles that don't get exercised driving a computer!
Cheers. PJ