Thanks Voicey. At 70% worn I may as well change it. When I got the car at just under 20k miles the clutch was 50%, give or take. Now, at 35k miles, it's at 70%. Assuming its first few years did involve a bit too much time in town traffic (both previous owners were London-based) it seems fair to assume that my use has been kinder to the clutch and that they *should* give 50-60k miles before needing changing. Seems a reasonable rule of thumb for prospective new owners to keep in mind when comparing mileage and prices.
Is there a consensus view on whether hard and fast changes are better for wear than more sedate driving (not that it would slow any of us down, I, sure!).