I’ve just ordered a mobile internet dongle from 3, through a very good offer I discovered through Martyn Lewis’s Money Saving Expert site.
The dongle (currently being advertised elsewhere for £50) is free, and the monthly rate is half the normal price:
1Gb per month is £5 monthly
5GB per month is £7.50 monthly
You have to get the deal through the web cashback site Quidco. You don’t have to join Quidco, but if you do you get further cashback (£12.50 for the 5Gb offer).