306 HDI or DTurbo as I said in PM - a bit of tuning potential if you so desire, economy and a good chassis.
If you want something readily available then the Astra is probably an option but most seem to be higher mileage. If driving pleasure is far from your highest priority you might be able to get a higher mileage Octavia SDI, probably ex taxi but comfy and lots of room, you can make a bit of money on a weekend if you hang around outside the nightclubs
I have a '99 106 diesel coming next month, low miles, 60mpg and so on but it's small and flimsy, I'm hoping to to swap it for a 306 diesel later or a Fabia vRS if my finances allow it.
As this is a financial move rather than one for pleasure it might also be worth browsing http://www.vegetableoildiesel.co.uk to explore which cars are able to run on veg oil to reduce running costs even more.