harman, what you can do with C4U (only on the phone) is say you will send a cheque for the deposit; that will hold the cottage for you and give you 3 days-ish to get some money together from somewhere and then you could pay by card.
No chance of a smaller deposit I'm afraid (I work there)
Also apols for you being unable to get through, this is the busiest time of year anyway plus the websites are playing up so people who would normally book online are ringing in as well.
It is worth hanging on though, it's only 1½p a minute on a Sunday and you will get through eventually - most people put their phones on speaker and go off and do something else, it's quite funny sometimes when we say hello as we'll hear someone galloping to get back to the phone before we give up on them!