I would have no hesitation in buying a second-hand diesel. I have a diesel Volvo which does about 40mpg average; my previous diesel was slighly better. Prior to that I had a petrol Vauxhall, similar performance (all 3 2.5 litre), which returned about 25 mpg.
12,000 miles at 40 mpg = 300 gals - at £5/gal = £1500.
12,000 miles at 25 mpg = 480 gals - at £5/gal = £2400 ie £900 a year more to run a petrol equivalent.
They can add a fair bit to RFL and diesel duty before you start losing out. In the meantime you get the benefit of greater distace between fill-ups etc. I can see no way they can make sudden large increases to diesel costs without upsetting large numbers of people - increases might come, but slowly.
Incidentally, it might not affect you but, in most countries on mainland Europe, diesel is around 20p/litre cheaper than petrol.