I watched that episode with the laughter in mind tonight. I see several places where the laughter did drown out the lines but my thought was that should not happen if it was canned laughter. They would have been careful not to not cover up the lines. I felt it was because the laugh was bigger or longer than they expected, so the pauses weren't long enough.
Peter Sallis talked about this in an interview on 30 Years of Last of the Summer Wine. The example he used was Destiny and Six Bananas (which was shot just the next season). When Foggy says he used Horlicks on the dart, Bill waited quite a while before he delivered Compo's line "That's supposed to make it sleep?" The laughter went on much longer than anticipated so you heard nothing of Compo's line and only saw his lips move. So they were trying to leave the gap for the laughter; they just weren't always successful.
I know they didn't have a laughter track for Getting Sam Home? Do know of any other's that didn't use one?