You're stuck with your 308 BlueHDi and with having to regularly add AdBlue, but it should be doing 60mpg plus. I wouldn't get another diesel, not just for reasons of potential future tax and restrictions, but also the high cost of maintaining the emissions equipment. BMW had some teething trouble with the 3-cylinder 1.5 petrol engine in the MINI Cooper, but seem to have sorted that out now, so that's the engine I'd go for.