I noticed that the url from quidco is
http://threestore.three.co.uk/dealsu...ECD001&id=1183 which for the £7.50 deal ends with offercode=18M3ECD001&id=1183 now, some interesting things occur when you alter some of the numbers in that url. ##M represents the months of the contract, however this cannot be changed from 18 months. #E stands for for GB allowance so 3 for 3GB (£7.50) and 7 for 7GB (£15) however if you change it to 1E you get the 1GB allowance for £5/month on 18month contract. And finally CD001 stands for the 3 usb modem, changing this value to CD002 gives you the ZTE USB Modem. i gues &id=1183 referred to affiliate network.
Therefore changing the end of the url to 18M1ECD002 gives you an 18 month contract with 1GB allowance per month for £5/month with the ZTE usb modem.
nb- I cannot be held responsible if 3 say that these offers are not valid or quidco fails to work.