I left Plusnet about 3 weeks ago and cancelled my direct debit at the same time since the payments are taken in advance each month. They then started emailing me because they couldn't take payment (for the service which I was no longer receiving).
I eventually got through to accounts and they sorted it out and said they would issue a refund for the overpayment from the last month. I just went to cash the cheque only to find that it is dated 2/Sept/2014 so the bank won't take it. Nice one plusnet.
Cheques have no expiration date however most UK banks will not accept a cheque older than six months. Where there is a dispute, a cheque remains legally valid in order to prove a debt for a period of six years, which is the Statute of Limitations.