We have a clause in our rental contract that we can't switch energy providers.

Do they? That might explain why they Customer Service woman tried to get me to pay by debit/credit card over the phone even though a cheque was in their system for full payment.

I've recently paid my Gas and Electricity bills (both British Gas) after receiving the final reminder. This advised that if the bills weren't paid within 10 days and a further reminder was sent, I would be charged £14 on both bills...to cover the cost of them sending the letter. This was received on a Friday and on the Monday I sent cheques for both accounts with payment in full. On the Tuesday (2nd Dec) I rang British Gas and advised that a payment had been sent. The Customer Service assistant told me that as it takes 14 days (!!) for a cheque to clear into the company's account 'they would probably still send letters and charge £14 for each' I was given the option of paying in full (again) by debit or credit card over the phone and the cheques would be credited to my account for future bills. £200! I asked if a note would be put on the system that the cheques had been received and was told that it would, but this wouldn't stop further overdue letters being sent...for a total of £28. This is exactly what has happened. The cheques have cleared my bank account, but I've received warnings letters threatening 'entry and disconnection' + the extra charges. Is this unfair trading terms?