I do this route at least monthly and anecdotally the late out from LHR/day back (239 and 238 I think) are always triples. Last year there were periods of 2 74's but over winter its pretty much three 777s.

They throw a 747 occasionaly on at the last minute for demand (Xmas for example) but you can't normally count on it. FWIW the BOS 777s aircraft have more than their fair share of small screens...