Tom is under-educated

I can't remember whether it was Further Group Theory or Number Theory that I took when I could have been taking Martingales and Financial Mathematics. Both have been useful in my life, but in this situation appear less so.

My bank wants to sell me the following deal:

* Let X be the current value of the FTSE 100 index
* Let Y be the value of the FTSE 100 index on 15 June 2010
* On 15 June 2006, they give me £3168
* On 15 June 2010, if Y<X, they give me £7000. If Y>X, they give me 0.75*((Y-X)/X) * £7000

All the figures for money the bank gives me are after tax.

This looks to me like a classic sort of option-pricing problem. How do I figure out what a fair price for the deal ought to be?

Current rate for a five-year bond is 4.0% after tax; I'm sure there's some sort of bond-lengthening derivative equivalent to 'I will pay you on 15 June 2010 the amount of money you'd have then if you'd bought the best-on-the-then-market 4-year bond in 15 June 2006', but I don't know what it would be called or what it's value is agreed to be.

Equally, I'm sure that long-term FTSE futures are a standard derivative, but I don't know how to find the relevant valuations.