I know this could be voted cold but its a reasonable offer.
O2 are giving a 50 quid high street voucher to spend on buying any 12 month sim only smart phone tariff. The 20 quid one is the best offer hence I have posted it.£50 quid from quidco as well.ie 100 quid back. 12 X20 =240.
If 100 quid back then £11.70 a month.Not bad at all.
Hope this helps someone