I'm the executor myself

My dad Dennis died on the July 4th.Phoenix Life continued to pay a pension annuity into his bank account after this date.Because of the onerous and bottomless pit of issued that had to be cleared after his death -funeral,registerong his death,probate-not to mention the grief that really only hit 2 months later-I overlooked these psyments. I decided to do what was the honest and decent thing and notify them of his death and they even demanded a death certificate which i sent. I wish I hadnt bothered!!-Today i found a letter from them demanding a cheque from me for £287 which they apparently "overpaid" him after he died. I am furious.I need this like a hole in the head.Is there any way I can legally refuse to pay?-I cant afford to pay this out.