As an update Dennis Turner's final match now seems to be Rochdale 4 Northampton 1 on 5th May 1979. As per AT's initial post I also had thought he hadn't refereed after an Ipswich-Arsenal game in March 1979 and had left the List a little early. However the referee profile for the Rochdale game makes clear that it was due to be his final match and that he had reached retirement age. This also surprised me as I was under the impression he was short of the retirement age in 1979 (profiles suggest he was 35 in 1969 so only 45 in 1979). It's possible the earlier profiles underestimated his age or the Rochdale programme heard he was retiring and assumed it was on age grounds. Whatever the case it seems we have a new final game for him.
Thank you LF for the update, I will amend the database. My information on why I thought Dennis retired after the game at Ipswich came from a RA colleague who was a linesman at the game and who suggested that Dennis was in bits when Bobby Robson (the Ipswich manager) came into the room because this was his last game due to work reasons. Perhaps his boss had a temporary change of heart but it wouldn't surprise me if Dennis did retire earlier than he needed to.
Edit - wasn't Dennis another FL retiree who went off to the USA to seek his fortune? If so, perhaps that made a difference to the date of his retirement from the FL in 1979, but would have precluded him from carrying on to the normal retirement age.