Question Author
Thank you all for taking the time to respond. I emailed Neil Cooke as suggested, and he had not known the site was down, and has said he will investigate. So, worth doing.
I have not yet used either Bookhopper or readitswapit, but wanted to try out Bookhopper because with their system, you submit books you have to swap, then can choose from all the other books on a credit system, rather than doing a direct one-to-one swap as with readitswapit. So you have a wider choice of books to receive. Sounds a good idea and I look forward to trying it out.
Thanks again for your help - this site is another great idea!