My advice would be to call them and to explain the errors of their ways, of which their appear to be at least two from reading your post:
1) My bet is, because the line+FTTC order (for UF2) was placed on their systems *after* the price went up, this is now the price they have applied to your 12 month UF2 contract. Whether this is right depends on what they said to you on the phone and what was agreed on the phone regarding this whole migration process from TalkTalk, which you say had to be done first (normal ADSL+line switch over before the joint line+FTTC package could be ordered). So I would challenge them on that.
2) The £15.98 line rental charge during the period where you were on their ADSL package is also clearly an error and they need to fix this up too.
Call them, and speak to someone human. Explain to them why you believe their charges are wrong (clearly, at least 2 above is wrong for sure) and 1 has probably occurred for the reasons I've explained above. If the agent you speak to is unable to help you or correct things, escalate to a line manager and have this sorted out.
I am surprised 2 above has happened to be honest - their billing system seems solid, but something has obviously gone south in your case. Call them, speak to someone human. I did when I first joined (to correct my Direct Debit date) and had no issues at all.
Edited by mixt (Sat 27-Sep-14 04:37:12)