I have a '99 diesel Skoda Oktavia which has done 64,000 miles. The timing belt was last changed by the original owner in 2005 at 36,000 miles. Is it ready for a new one or could I get another 20,000 odd miles out of this one? The car will probably be dead by then though (engine is still fine but other bits are starting to go!).

The manual says change them after 60,000 miles so the first one was well inside the manufacturers recommendation.

If it makes any difference we got the car from a relative and know the car from new and the original owner was a very careful driver, and I've never ragged it in the 3 years I've had it.

It's normally change it every X miles or Y years whatever occurs first.

I've got the VAG diesel in my Ibiza so it's probably a similar engine to your Skoda & mine is 60k miles or 5 years, I think. Although it might be 4.
Funnily enough mine's on ~117k miles so will need doing in about 5 weeks or so.

How the car is driven (ie. ragged or driven like a saint) makes very little difference, I believe. The cam belt will degrade over time & that's why it also has a time limit.
I'd get it done, unless you can afford to have the engine 'rebuilt' if it goes wrong or replace it.

Cambelt on the 1.9TDi isn't that expensive – I think the last time I had mine changed it was £270 including a service.

Do before it's due. I have a Y reg. Transporter 102 TDi which has done almost 100k. The cambelt was changed at 50k (vs 60k advised) & I was just about to get it done again……………….when it broke & I needed a new engine – I could have bought a bike for what it will cost me!

Age or time, whichever comes first. If you are a Gambler leave it longer.

Cambelts dont know abuse.

How long in years before recommended?

Pump and belt change would be circa £300-£400. Your car is worth about a grand? (not checked though). Although if you ran it for another two years and scrapped it £300 outlay is pretty good.