Thanks for the comment. I posted the same question on the "other" forum and I selectively quote from some of the replies. This could possibly throw more light onto the topic and generate some more debate:

"One of the men I game with remembered reading an account of an old Guard artillery sargent from the 1840 who wrote that they (the crew)watched the cav go over the hill as the cav crested they parted so even tho the artillery could not see the square they had an aim point so when the cav came down the artillery resumed firing.This explains British accounts that they could not figure out where the artillery was coming from."

"Looking westward from Wellington's crossroads one can see a bank about 6' high on the Northern, i.e. rearward side of the road, which on the forward side has been scraped away to build the mound. The road itself has not moved, but it ran through the famous hollow way which is no longer there.Thus it is unlikely that French gunners were able to see much of the Allied squares (though colour stands may have shown), but as ...... explains they would have been stupid not to take their clue from the cavalry's behaviour. Certainly after the first attempted charge, any hint from the re-forming survivors that "the redcoat squares are just behind the crest" should have been enough to make them lay on ricochet fire."

"I also read that the French artillery basicly fired where the cav parted."

Best
Fred