From Martin Bond, 99 Sqn historian, in an e-mail to me today:

Talking to 99ers about the Ennis incident, they ascribe his loss to cutting a corner on the return route and being jumped by an Oscar which itself was destroyed by fire from the rear gunner. I met Ennis’ brother many years ago but sadly do not have contact details. Policy at the time was to remove as much crash debris as possible (as well as any human remains) and re-use any re-usable bits. There may not be much left to recover. This was the role of the Repair and Salvage Units to which my late father was seconded during his spell away from 99 Sqdrn.

It’s not proof that return fire did down the Oscar, but perhaps it is true.