I get really good speed in the day until about 5 pm, after that and at weekends the speed dips and the latency increases. It's due to the exchanges being over subscribed, which is beyond the control of O2 as BT manage the wholesale customers. I pay £12.50 a month for O2 Access which is a lot cheaper than anything else I can get (no LLU providers here) and at least O2 don't make things worse by traffic shaping.