Really hoping somebody can help, I love the game but I've been stopped by a bug.

It's on the 'Tactical Thinking' mission where I have to wipe out the Railroad and recover P.A.M from beneath Old North Church.
One of the tasks is to kill Deacon, but when I do it doesn't register that he's dead. By that I mean he's lying on the floor and I can take items from him but the yellow objective marker stays over his head and it won't tick him off the list of targets.
Unfortunately, for some unknown reason my last save (auto or quicksave) appears to be a long time ago.

I've tried quick saving and reloading and also leaving and re-entering the area but he's still lying there the same.

Any suggestions? I can't continue otherwise.

(PS - The post title should have read 'bug', not 'but'. Autocorrect!)