Daveyboywonder – I still can't see why you would want to change.
My wife has a diesel Polo, all be it only a 1.4 and there is no way I would want to do 25k + miles a year in it, its just not a comfortable enough car, and it doesnt do a great deal more mpg, plus has a MUCH smaller fuel tank. I can assure you that when you have been used to getting 600+ miles from a tank, having to fill up after 350-400 miles will make a big difference. Oh also Polo's are extremly dull to drive and IMO the build quality is quite shocking.
How old is your Mondeo, and when did you buy it? The Mondeo depreciates at a greater rate than most cars, so if you have had it from anything like new it will have already depreciated big time regardless of putting more miles on it, hence why I paid £4k for mine and get a 4 year old one with all the toys.