The death of a Philip Heaney is recorded in the index for St Helier cpvering the period from Oct 1979 to April 1981. It is Volume 80 and is on page 75 which would make it towards the end of that time frame.
Jersey Archive has a copy of the will of a Philip Heaney of Mont Plaisir Guest House, Midvale Rd, St Helier and formerly of Overdale Hospital. Probate was granted in Jan 1981.
No guarantee that this is your grandfather but you can get a copy of the will from the Archive. You could try messaging the Registry Office,sir@gov.je, for details from the certificate - make sure you give them the details of the index or they will charge you a search fee.