12 miles, really?

- Cycle it

Firstly you do
not want a diesel, it would barely reach full temp by the time you get there. The DPF would easily get blocked, you want a petrol for around town only use

Any change in car is likely going to cost you a 4 figure sum, but lets be super generous here and say it costs you £750 in total to switch. Thats insurance, other paperwork, loss in depreciation from purchase, cost of new car ect. So a long shot

Lets say your cost per mile really went down. Quick Google suggest 22mpg from 2.7d Jeep? Think yours is that? Not sure what you really get...
30 mpg in a petrol are is ~ 19.34p a mile.
30 mpg in a diesel is ~ 20.42p a mile.
22 mpg in a diesel is ~ 0.27p mile.
Lets say you got a little Fiat 500 Twin Air thing, they "say" 70mpg, but lets be realistic and say 45mpg nocking around town. Or even 50mpg. 50mpg in a petrol (long shot!) is around 11.69p a mile, (almost same PPM as my 2.2 LPG) so about as cheap as you can get, with diesel being more at the pump.
0.27ppm (Jeep) minus 0.11p (twin air) = 0.16ppm mile saving.
So best you can save, really is 0.16p a mile, (which again is a huge saving in reality) take that £750 back in... you need to do 4,687 miles to get back just £750. Costs you £1.5k to switch? Around 10k miles! Not a problem when you do big miles, but not your kind of commute

Based on a 12 mile commute & around 253 working days in year (excluding your holidays) you are looking at around 18 months to pay back a £750 outlay in a different car. 3 years or so, for £1.5k cost to switch.
It's worth considering that the cost at the pump, although is the most 'visible' cost, there are many others
