I did eventually phone them again, and did get offered a slightly better deal.
£20pm inc line rental (with the option of £30 discount for 12 months LRS)
18 month contract
The retentions guy confirmed what the previous guy had said, they weren't able to match the new customer offer (which factoring in the £50 PN cashback is an £86 difference).
However the deal they were willing to offer me on a switch to 38 Mb fibre was £23pm ilr, which is the same as their new customer fibre deal.
Edited by gordon_k (Mon 28-Nov-16 13:40:55)