Sim only tariff from Vodafone, currently giving £120 cashback on quidco, comparable to the o2 simplicity cashback deal that is currently running red hot - although on this one theres a possible £120 back instead of £100 on the o2 so for some people this might well work out better. (depending if vodas tariffs suit your useage better than o2)
You can also get £100 cashback on Vodafones £20 a month sim only tariffs and £70 a month on their £15 a month tariffs.
Some example deals:
£25 = SIM only - 1200 mins, 3000 texts, Unltd landline call, 500MB mobile data
would give £120 cashback so effectively £15 per month
£20 = iPhone Sim Only, 600 mins, 3000 texts, 1GB web and wi-fi
would give £100 cashback so effectively £11.66 per month