I test drove both the petrol & diesel Soul and was very surprised how much difference there was between them and I'd view the £1000 premium for the diesel worth it even if there wasn't an additional benefit of better MPG. Gone are the days when the diesel was the sluggish noisy option.
However, our Soul 2 CRDi has now done 19,000 miles and we only get around 43-46mpg on average which is disappointing. On the plus side, it is quite lively and is good too at motorway speeds. A 1.6 engine is the smallest I've ever owned. Yet, I'm quite happy with the performance from it and rarely wish for more. Most of my previous cars have been 2.5 ltr petrols and larger. Our current second car is a Saab 9-5 2.3 turbo estate. It sounds like I've got a heavy right foot, but I try to drive economically. Just that 80% of the time the driving is school runs in country b-roads and the rest is motorway, where I generally keep up with the rest.
I'm writing this to say.. don't expect great MPG - the diesel doesn't get near the sales brochure MPG in the real world. I'm sure you could if you tried really hard, but I'd bet that would be to the annoyance of the others behind you.