Does anyone know what the chances of having £250,000 and something else at the end if £250,000 isn't in your box?
Or what about two particular amounts, say £250,000 and £100,000?

If you don't have £250,000 in your box, it's a 1 in 21 (or 20/1) chance it will be one of your last two.

If you don't know whether or not you have it, it's a 2 in 22 (or 10/1) chance.

Having any particular pair of amounts as the last two is 1 in 231 (230/1) chance, assuming you don't know what is in your box.