Hi mate - I remember your old RT!
First of all, the good news is that youre in a good position in terms of retained value wih the Mini.
You might not think so after having spent so much, but £6k is a lot of money and Id encourage you to spend even less and keep a few grand in your back pocket. You could sell the Mini, settle a large part of the finance on it and then get yourself something "new" with the rest. Spending a little less on the car will allow you to be more flexible with the insurance too.
Im thinking that because you got a new car in the first place that reliability and just being able to get into a car and drive it with no possiblities of a breakdown are important to you? If thats the case then you may want to rule out a lot of nice hot hatches like the 16V or Williams. But the fact remains that the 16V in particular is a very cheap car to buy nowadays and if you get one over 10 years old it really affects the insurance too (Group 12 as Im sure you know).
I think that the Zetec-S is a capable enough Mini replacement and probably as offers as painless a transition as possible from the handling/speed of your Mini. However, like I say, £6k is a lot of money and Id be unhappy losing a lot of that in depreciation on a Zetec-S.
My logic for this is that I recently bought a mint Willy for £4700 (plus £700 to insure, 1 year NCB, 23 years old) with 54k on the clock and I think thats more like it in terms of VFM and minimising the depreciation. From another perspective, I also drive my mums 1997 Volvo S40 - a 2.0 with electric heated leather, cruise, CD stacker, alloys, electrics etc. OK, so its not the most exciting car ever, but its really good at clocking up big miles in comfort and would probably go for about £5k these days (£20k new).
So, spend less and get more VFM my man! Forget this Zetec-S nonsense and get your thinking cap on!
Travelling around without car is easier than most of us think, so sell the Mini and then see how much dosh you get from that after settling some of the finance. Good luck!