I've just been out driving my wife's Matiz, and I've decided that the brakes are just not good enough.

First of all I reckon it needs a brake fluid change & re-bleed, but there seems to be an issue with the ABS too. When I brake hard I can hear a soft clicking noise (from the ABS controller, probably) and I can feel a slight 'ABS buzz' coming back through the pedal. I'm guessing that if a sensor has gone then the ABS warning light would come on (apart from when I first turn on the ignition, with all the other warning lights!) and it doesn't, so I suspect that it may be a wheel speed ring that's out of order in some way (rusty, broken, etc.).

I'm also guessing that if the ABS thinks that one wheel has stopped then it will degrade the braking for all wheels.

Does this sound like a reasonable analysis?

Does the ABS activate only at low speed, e.g. just as you pull to a stop? If so then this can be caused by a defective sensor without an error code being thrown, though a cracked reluctor ring is also a common fault.

No - it happens at any time I'm doing significant braking.

I've done a bit of googling, and it seems that cracked reluctor rings spinning round is a known issue. The front rings are mounted on the CV joints, which will be a PITA to investigate, but the back ones are mounted in the brake drum - I'm hoping it's one of the back ones!

Apparently a failing front wheel bearing can throw the abs computer, so I'll probably check that too...

Cracked reluctor ring, the reluctor ring is heat shrunk on to the cv joint, water gets under the ring and rust forms causing the ring to split.