I checked the cancellation page and apparently I'm paying 16.99 a month with a £9 a month discount. So cancellation is £169.99
I phoned plusnet again and tried the cancellation part.
Again I was told that 38mb unlimited fibre in my area, a non low cost one, is £23.99. I could upgrade the rest of my contract to that £23.99 but would lose the £9 a month discount. There was no chance of a new contract and writing off the old one.
All prices don't include the line rental at 15.99.
AS im on adsl2 just now at 13mb, I think I'll just hold fire until the contract dies in december.