Three are pretty good for mobile data Lizzie - they only have a 3G (2100) network, however, although it has the best coverage of the 3G network.
You can get a 1month/1GB SIM for about £8.00 on eBay or a 3month/2GB SIM for about £14.00 on eBay or Amazon.
Hope this info helps / Steve
[size=2]Steve Gold
PO Box 1014, Sheffield S10 5YG, UK
Home mobie: Telefonica O2
Other UK mobiles: 3, Vodafone, Virgin
Foreign SIMs: Toggle (multi); Germany (Fonic); Poland (Orange PL);
Skype: stevewgold