The title says it all: Nokia 3109 on 12 months Combi 30 from T-mobile, 11 month free via cashback + £20 through Quidco will give you 300 cross-network minutes, 200 texts and a free (basic) phone for just a tenner per year! If this is not a good deal, tell me what is a good one :)
Some personal experience feedback on top: ordered this phone yesterday afternoon and have been woken up by the delivery this morning. One thing to know is that they don't port your old number, but instead you have to phone up T-Mobile and they'll do it for you (took me less then 10 mins, don't forget to get your PAC code first). My previous phone was from BuyMobilePhones too, cashback is straightforward with them if you're smart enough to post the right bills to them 4 times a year. Their only downside is that they will ignore your emails (if you wish to send them any), but client service via phone is acceptable.
Before you notice: this deal is better then the one with Nokia 1680 on 12mth contract with 12 mth free via cashback from the same website - because they never had 1680 in stock since they announced that deal and are not planning to get more stock either.