This deal was posted a few months back, but I note that TCB are now offering £80 cashback for "broadband 76" orders. Get it while you still can. There's also 50% off a new line (£24) if you need it provisioned for FTTC, but I've just switched from sky without any additional charges.
Quidco are offering slightly less at £50.
It's £31 if you are not a Vodafone customer, however you can order a free PAYG sim, or pick one up at the supermarket for £1. Just register it without topping up then you'll have instant access to the £28 per month.
Deduct from that the £80 TCB over the 18 months and the effective price is £23.56, which is by far and away the cheapest 76 meg fibre deal available through the copper wires right now.