If you are in a market 3 exchange the PlusNet Value and Evenings and Weekends package is £16.94 and the PlusNet Value and Talk Anytime combination is £21.94 per month. If you are not on a market 3 exchange, I think it's £6 more after the first 3 months. It is an 18 month contract.
We have Plusnet and have to say that while it is cheaper than some, the customer service is pretty poor, I've had to call them on a few occasions and they have never been able to help (neither were they keen to help) and so I've ended up spending loads on computer repair type people.
O2 are getting really good reviews for broadband service and quality of line seem great. I'm just in the process of changing my broadband from by. Cost is about £10 per month but that is discounted as I'm an o2 mobile contract. I think it's about 12 or 15 normally . Hope this helps