Certainly ring the COT as suggested. I hate the way ISPs (along with suppliers of many other services) give much deeper incentives for new customers than they do for loyal customers; and that loyal customers have to fish to get any incentives.
I don't know what Plusnet will offer you for retention; I'm just about to move to Plusnet. Part of the reason was their reputation for service (already looking a little tarnished?), their reasonable standard prices, and the offer of a 40/20 service. But I must admit that around £60 off the first 6 months broadband and £100 cashback influenced me as well. BT retentions almost matched Plusnet standard prices, but didn't attempt to offer anything that makes up for those other sweeteners.
BT Infinity 2, moving to PlusNet (26th Sept if all goes well)
Edited by StephenTodd (Wed 24-Sep-14 09:46:18)