Contact the company's customer service department to explain your problem, as they may be able to sort it out for you straight away.
If not, make a formal complaint to the company. Consider doing this in writing, although you can do it by phone or email.
If it's still not resolved, ask for a deadlock letter; this is confirmation from your provider that they've been unable to resolve your complaint themselves, or that they don't feel there's any more they need to do.
Once you have a deadlock letter (or if 8 weeks have passed since you formally complained), you can then go to an Alternative Dispute Resolution (ADR) scheme. This is essentially a middleman who can step in to help resolve your complaint.
There are two ADR schemes for the communications industry. Both are independent and approved by Ofcom:
Your provider has to be a member of one of the two schemes. To find out which, ask your provider or refer to the ADR schemes' websites.
When you escalate your case to an ADR, make sure you give them all the information you can, including copies of correspondence.
Once they've collected all the information they need, they'll pass your case to an independent adjudicator, who will usually make their final decision within 6 weeks.
If you accept the ADR's decision within 6 weeks, your provider will have to carry out the actions specified.