Shorter journeys however i'll do around 10 - 12k per year, I currently drive a ford fiesta so im looking for something with more room inside and a bigger boot. Saloon to small estate.
Thanks
With use and mileage like above you're probably much better off going for a petrol engined car.
Your annual mileage is only just about average, and short journeys are the worst sort for a diesel engine as they don't get time to adequately warm up.
Turbo diesel engines are much more complicated than NA petrol engines, and therefore have the potential for more, and more expensive, things to go wrong with them. Coupled with the price of diesel being, on average, 6.6% higher than petrol, and the higher initial purchase cost of the diesel car you'll have to do many more miles before you'll see your money back.
Average 35mpg in a petrol costs £1414.49 for 10,000 miles in fuel
Average 50mpg in a diesel costs £1055.60 for 10,000 miles in fuel
Grand total saving of only £358.89 / year
If you're mainly doing short journeys I think you'll really struggle to get to 50mpg average (I currently drive an E91 320d, I do 25K miles per year, most of my journeys are 125+ miles, and I only just get over 52mpg average). You're more likely to see low-to-mid 40s.
If you managed 45mpg your fuel cost for the diesel will be only £241.60 cheaper per year than the petrol car.
Either saving could be completely wiped out by something going wrong in the diesel (e.g. DPF issue, turbo problem, injector problem).