Can any of the probability whizzes here tell me how hard this would be to pull off?

The problem is there's no real probability of determining whatever amount being won. How would, for example, you tell me the probability of the £250,000 [don't say 0, please!!] If everyone went to the end, it'd be 1 in 22, but it's not: the only real way to do it would be with empirical data. Go through the archives, find the proportion if sub-22 wins and work out that proportion to the power of 22. It will probably be very big, though. I don't have the data with me, but unless there's a phenomenal proportion of under-£20k wins, it'll be very small. Even if the proportion of under-£20k wins is 0.9 [i.e. 90% of people left with under 20k] then the probably would still be less than 1%

Hope not!

The future of mega wins lies with the ones I believe can win big and that would be Jade and Tommi

With the bunch on the wings now I'd say its the nearest thing to a certainty this side of the sun rising tomorrow morning.

Even if they get in a good position banker man isnt offering over 20k as he got the rule he doesnt want them to win over 20k, and for theones who went for it, stoney,harry, ian, diane, had terrible rounds after the near 20k offer!

Short answer - no. Given the bankers offers vary from show to show, there wouldn't be a way of determining the probability of a win from any show.