Laura, this is taken from BT's web site;
It lists an 0800 number which will be free from a phone box or a friends BT land line.
Remember it's not free from your mobile
Personally I'd put it in writing too.
This code of practice link should work
44. We will do everything we can to follow our Code of Practice at all times. However, despite our best efforts, things can sometimes go wrong- and if they do, we want to know so we can put them right as quickly as possible. So, if you have a complaint about our sales or marketing activity, please call us on the appropriate number:
If you are a residential customer please phone 0800 800 150 Monday to Saturday, 8am to 8pm.
If you are a business customer please phone 0800 400 400 Monday to Saturday, 8am to 6pm.
Our customer service advisers will try to solve the problem while you are still on the line.
45. If you prefer to put your complaint in writing, please address it to the Customer Service Manager, BT Customer Correspondence Centre, Durham, DH98 1BT. We will normally respond to your letter by phone but will gladly confirm any particular points in writing, if you wish.