Last payment should be back but didn't

[Alfred+Poniedzial]

i close my sky broadband when contract was end but they are told me i need to pay one month more even boardband it's close and they are return this £40 me back.  So long time i was waiting for return it and ask them. Finally answer me ,,we send you cheque.  I received cheque with mistake ( I'm male and there was MRS) and i can't realise this cheque. In chat told me i need to go post office and there are have sky address. I was but in post office told me i need address and label. Never answer to emails and i can't use any more chat. Why?!