You chaps were of course wildly correct as always. I found that flat section, but that was no help... so I ended up removing the circlip and then just pulling the entire cylinder up through the engine bay, hose still attached. If you're quick, you don't lose any juice, even if the cylinder itself is leaky. Up there it was a simple matter of putting the cylinder in a vice and removing the hose - if you're above the level of the reservoir, you don't even lose a drop of fluid (mine hose is now cable tied to the accelerator cable).
On inspection, it looks like the degradation of the dust boot allowed water and crap inside, which as you said, caused some corrosion. Sanding everything back, it doesn't look too bad (a bit of very mild internal pitting, but i suspect with everything smooth and a new washer, I can live with that).
With everything cleaned up and wire brushed, it's not looking too bad - is there any reason I can't give the cylinder a coat of left-over wheel lacquer while it's out? I'll see if I can grab a seal kit, and then reinstall and bleed the system.
Thanks again for the advice guys!
A good wire brushing goes a long way:
If you look *really* closely, you can see at the top of the washer where the corrosion had worn a flat spot - the cause of the (albeit fairly slow) leak: