Update, FWIW.
BT have not yet unbundled the line where we are moving so TalkTalk cannot provide their service but said I could pay them £14 on top of the £6.99 for free calls to get the BT broadband.
When I said that I may as well go with BT as in that case it would be cheaper they put me through to the cancellation team who gave me a credit and monthly discount which would effectively make the cost the same as BTs cheapest broadband and phone package, for a 12mth contract instead of 18.
Also they give me 40Gb usage Vs BT 20Gb, no fees for going over the limit Vs BT+£1/Gb, free evenings and weekend calls Vs BT just weekends and free international call saver (worth £2/mth with TalkTalk).
Couldn't really say no and now I have one of those super BT lines that everyone raves about!