Take the £25
People seem to be highlighting the wrong part of this..
From their T&C's:
Wherever possible we will accept your order to buy the goods of the specification and description at the price indicated by e-mail and in which case there will be a concluded agreement between you and us.
They should be highlighting the 'Wherever possible' part...
You've said you don't need the CPU, they've offered you either a £25 credit or the product for £100 less than its current price, so, you either get £25 for nothing, and when I say nothing, that's not really true anymore, you have spent your time creating this thread, so, work out what your hourly rate is based on that, or you get a CPU you don't need for less than you should do, in which case you have still got a deal, or, you flip it and then make whatever difference you can between the cost and the sale price...