Right, here we go Scott 
Assumptions:
- I know fuel cost won't be the same for 3 years but lets assume the gap stays the same
- As above for road tax
- I've been generous with the petrol MPG..
- Cost new was taken from the ford.co.uk configurator for a 1.6 petrol & diesel Zetec-S and is list price with no extras or haggling!

Nath,
Here's some real world figures for you to put into your spreadsheet. Filled up an almost empty tank yesterday. Odometer was 341.1 miles from full, with 5 miles left until 'empty'. Put in 41.87 litres at £1.319 per litre (Shell, Exeter), which came to £55.23. So my 1.6 118hp petrol engine costs £0.16 per mile to run based on a my average driving routes of 20% urban, 80% motorway. I don't exceed 70 on the m-way and change up on the green light in town. But no, that doesn't mean I drive like a granny, and acceleration is still 'decent' even when driving economically like this as it pulls reasonably well at relatively low speeds in high gear.