Check the warranty. 7000 is quite a lot for a demo car, especially in the UK. Demo cars might not only be the variety they use for going around with the car salesman (who does usually encourage you to gun the engine) but also might have been loaned out for a night or two to "test out the car". All things to consider. But if you have a warranty that you can live with, then it might not be a bad deal. Since this is essentially a used car for all intensive purposes, I would look at what other '05 Jazz with 7000 miles go for in the market.
Now rental cars!!!!! NEVER EVER EVER buy a used rental car, no matter how nice it looks. I have driven rental cars for years, and between myself and co-workers, people behave totally differently with a rental car versus their own car.