Yes I have considered diesel but [without boring you with the calculations] based on Seat official fuel consumption figures I would only save £500 over 100,000 miles.
This is because the cost of buying the diesel in the first place is considerably more than the 1.4. Over the 100,000 miles the extra dosh would be sat in my savings account earning interest too - so it would actually be less than £500.
My 1.4 MK3 has been reliable and the fact that the 3 year warranty covers me for the first 60,000 miles - reliability does not become an issue.
Another thought is that it is everything around the engine that has gone wrong on my MK3 (tyres,exhaust,starter motor,sensor,front shocks,brakes etc)
So I maintain my opinion that on a small car like the beeza it is not worth buying a diesel ... on larger cars I can see that it could be worth it and if you do a lot of towing etc.