Some useful information:

Sales such as these are now covered under the Distance Selling Regulations 2000, which includes, amongst other rights, the right of the consumer to cancel the order within 7 working days without needing to give a reason.

See http://www.dti.gov.uk/ccp/topics1/guide/distsell.htm

So if they fail to deliver within their specified period inform them that you are cancelling your order as per your rights and there's damn all they can do about it.

Thanks, Graham, that's useful. But I don't really mind about the time it takes - I'm just about old enough to have got over the worst excesses of the desire for instant gratification - but I just wish they'd be straight up about it.

Have you thought of stirring things up by writing to one of the customer complaint pages that most newspapers seem to run? In the one I know about, in the Guardian, they seem to be forever dealing with complaints about firms who ignore customers who are trying to get something or other straightened out with the firm.