Petrol.

Modern engines are (relatively) clean in terms of their emissions.
Short stop start journeys are where Petrol is less efficient. They use much more fuel under these conditions and the Catalytic converter (which is what cleans up petrol car exhausts) takes several minutes to warm up to efficient opperating temeprature. So, from an environmental and economy point of view. A petrol engined car is at its best on longer journeys. Regular servicing of Petrol Cars is critical to maintining their economy and enviromental standards.

Diesel.

Modern Turbo charged and intercooled diesel engines with electronic engine management systems are (reletively) clean. Particulates have been reduced dramaticaly and with modern Low Sulphur fuels which are now the norm in the UK, even further to a point where they are largely insignificant compared with other sources of particulate emissions in the environment. These engines are quick to to achieve their maximum efficiency and have their biggest advantage over Petrol Cars in town and city stop go driving. (Look at how many Taxi`s are diesel powered!.)
This is because of the Power characteristics of Diesel engines which produce more useful power at lower "revs" so the engines have to be driven less "hard" in city conditions to keep up with trafic. In open road and long distance driving a Diesel looses SOME of its economy advantage over Petrol but still should return between 10 and 20% better figures than a Petrol equivalent. Yes, there are small Petrol Cars which, if driven very gently, will give 40-50 MPG on a run. But speak to drivers or small Diesels and their TYPICAL fuel consumption will be between 45 and 50+ MPG no matter how and where they drive, town, Motorway, mixed......it makes very little difference. I had these figures from an "old generation" Peugeot 205 Diesel in the mid 1980`s (all carefully documented) and so I am not exagerating here at all. I must confess however that I did once manage to get the consumption down to 38 mpg.............towing a Caravan at 60 on the motorway!! :-) A modern equivalent of that car should easily be giving 50+ mpg and probably much better performance as well and a GREAT deal more cleanly.
Service intervals on Diesels are now simmilar to most Petrol vehicles and the service is a much simpler affair.........usually just oil and filter changes as with Petrol cars........but no ignition system and spark plugs to be replaced or need adjustment of course! (Another factor which improves Diesels "whole life" economy and environmetal credentials vs petrol.)

I am consious that my opinions only relate to UK driving conditions, model availability and costs. But for me it would be Diesel every time unless there were some specific need that would preclude it. For example, needing an Automatic Gearbox in a small car.....they just dont sell em in the UK with a diesel engine! or a very low annual milage.

In my view.........for the type of driving that you do Ed, a modern Diesel made in the last 5 years, would suit you very well. You do enough miles for the economy adavantage to be significant and would be making less contribution to global warming as a result. I think you would also enjoy the driving experience of Diesel cars these days. Petrol vehicles, in order to get those economy figures and environmental benefits, have become gutless and harder work to drive in normal conditions.....unless you put your foot down, and then of course you loose those benefits big style!! Hope you find something to meet your needs........come back and tell us which way you have decided.