Fairly well hidden, although it is available via a ring-back service, the Amazon customer service number is 0800 2796620 **now 0800 4961081**.

Home Delivery Network don't have a number anywhere on their website. Their telephone number is 0870 1545400.

** 4 December 2009 **
From the comments it seems like nothing has changed in the 3 years since I made this post. If you get any problems your first course of action should be to phone the retailer. Hassle them every day until you receive your delivery. If you spend time chasing HDNL yourself - phoning depots and customer service or even driving to the depot - the retailer gets off easy. And if they start putting the pressure on then perhaps things will change...

If you must contact HDNL then the latest advice, from a comment left here, is:

"dial the 0871 977 0800 number, it initially says it's an automated service but if you let it go through the options and do nothing you get the option to press * for the menu, if you do that and go through the options again you get to press * again to speak to a customer services advisor."