It is also worth trying your local independent rescue centres, to see if your dog has found its way there. It might have been picked up as a stray and taken to a local pound, then rescued by a local centre when its 7 days were up.
You could try places like these
Evesham Greyhound and Lurcher Rescue
Celia Cross Greyhound & Lurcher Rescue
Ultimate Animals- RESCUE A UK PET
Sheya Greyhound and Lurcher Rescue, - based in Manchester, but covering most of the country from Dorset to Durham
DOG LOST UK - This site, in addition to registering details of your dog and allowing you to make contact with others in a similar situation, also has an option of an automatic poster delivery service. They have a data base of over 15,000 organisations nationwide and are able to fax and email your missing dog poster using their unique efax system to every vet, dog warden, kennels, pet shops and fellow helpers in a zone 30 miles all around the place you last saw your dog.
This obviously costs money to send and they do ask you to cover the £36.00 cost of the mailing. However pensioners and people on low income will not be charged as they have owners who have successfully used the delivery service willing to sponsor these cases.