Mmm....this is a strange one. I was the one, if you read back a few posts, who made the original statement that the shot of Mike looking up from the beer cellar was taking from the original footage (filmed with Lennard Pearce) and started this thread-within-a-thread. I took this information from Ken McDonald (Mike) himself, who said this was the case in 'The Story of Only Fools and Horses' book.
But, due to the poster above who said he was convinced the shot in the episode was filmed with Albert and not Grandad, I looked again. Despite what wiki says, I think it is indeed Albert. I have freeze-framed the shot and you can clearly see Albert's coat and beard, and not the edge of Grandad's coat. They are of slightly different colour. If you freeze-frame it at the point where Mike says 'I want...' (as in 'I want a word with you, Trotter!'), the white beard is visible in the very top corner of shot.
I don't know if we'll ever know for definite (and I guess it doesn't really matter all that much), but I think the freeze-frame here is as good a proof as it's possible to get that the scene was indeed re-filmed with Albert, and that is is not the original Grandad footage that was used, despite there being claims to the contrary.