Not really. Generally speaking, episodes are not filmed in order. Rather, different scenes are filmed at different times. Last Post was the first episode they started working on that year, with the Dover and other stuff done on location. But then they went back to Holmfirth and filmed various scenes there that included Last Post as well as the first three episodes of that series. Last Post was mostly completed when Bill Owen died, but those other episodes were not and had to be salvaged later. That was when they used some old unused footage, body doubles, and a few voice dubs to try and complete them. How much varies on the episode, but the first two were largely complete. The third is where they really had to stretch things. If I recall correctly, Bill Owen's actual last filmed scene was from one of those episodes, not from Last Post.
That, at least, is what Alan Bell says in his book.