Their mistake, they should pay for it in full, not 1/2 as "gesture of good will".
I would write to them (e-mail/fax? if you could get a response straight away?) and tell them that as time is of the essence, you will pay for it straight away, but that you do NOT accept 1/2 as being good enough as a GOGW, that you expect them to reimburse you in full for THEIR mistake, and that frankly, not only you'd expect them to pay for their mistake, but to throw in something extra, THAT would be an appropriate GOGW. Cheeky beggars.

__________________
Barclays: Won ~ NatWest: Won ~ Halifax (x2): Won ~ FNMF: Won ~ Barclaycard: Won ~ GHD: Won ~ Grattan: Won ~ GE Money: Won ~ Capital One: Won ~ Land of Leather: Won.
*
This wonderful site relies on donations to stay open.
Help them help us, whether it's £1 or £1000, it all helps.