I am getting very confused messages. Help! Anyone with a definitive answer? I'm willing to expand on what has happened. DD

Hello, I recently offered British Gas £64.00 in full and final settlement of an £84.38 bill. There is good solid reasoning behind this. I wrote to them enclosing my cheque and made it clear it was in full and final settlement and that all they had to do to accept my offer was cash the cheque. They did cash it, but they are now chasing me for the £20.38. I believe I made an offer that they accepted and the debt is now paid in full. Any comments?? Regards DD