Basics are that I've just bought a used car from a Ford dealership... I've part exchanged my current car after they offered �0„5XXX in part exchange.

I've agreed the price, signed all the transfer documents online, paid the difference and for car tax, and delivery - all signed, sealed and done.

2 hours later they have phoned back and said that they have made a mistake with the part-exchange valuation and they can now only offer me �0„5YYY (half the amount of �0„5XXX) due to it previously having been an MOD-owned car (which I fully disclosed to them) - they are insisting that it's all null and void and that the whole deal is off, so I'm just wondering if I have a leg to stand out if I play hardball and say that it's all been signed and paid for?

Thanks.