mat_fenwick wrote:
I would add to the above list a hydraulic pump belt, as if that goes you're fairly stuck..
Very definitely - a couple of weeks after we swapped from the white estate to the grey one, my wife was in UK when the belt went when she was on her way to the ferry to come home. Naturally, I'd swapped all the parts and tools over from one to the other - except, as it turned out, the pump belt which was still in the white one.

If she'd had the spare belt, she could have changed it herself and made the ferry. As it was she had to leave the car in a motorway services and fix it the next day.
Thinking about it, that remains the only time in 12 years and at least 150,000 miles in various BXs that it's not been possible to get moving again with what's on board.
Basically I carry anything that doesn't weigh a lot or get in the way. You can fit a fair bit in alongside the jack inside the spare wheel, then there's the underseat space.
Off the top of my head:
Bulb kit
Belts (alternator and HP)
Some low pressure hose.
Front strut return pipe.
One of the steel sections the above fits to, on the grounds I can cannibalise it to connect bits of octopus if necessary. Couple of short bits of HP pipe with swages at each end would do the same job but I keep forgetting to make some)
Stop 'n' Go Tire Plugger kit (google it - awesome) when I remember to swap it over from the BMW where it usually lives).
Umbrella (not breakdown-specific, it's there anyway).
Something to kneel on if necessary.
Lightweight waterproof jacket - preferably violently visible.
Gaffer tape
Cable ties of varying sizes
A few jubilee clips ditto
One pair of part-used front brake pads - only needed them once, but once was enough.
Odd bits of wire, fuses, sticky tape etc.
Keep meaning to put a spare clutch and throttle cable in - I've got them, so why not? (Except changing the clutch cable on a TD seems to be impossible, of course).
An LED head torch.
Some vinyl/latex gloves.
The odd bit of rag.
Oh, and some toools obviously – chosen for what's likely to be needed, rather than a random selection.
Might seem a lot of stuff, but a lot of the miles we do are thousand-mile trips over two or three days, involving a ferry journey, so the implications of a breakdown are rather more complicated than just getting towed home 20 miles.