I just wanted to say a huge thank you to everyone that sent private messages with lots of genuine tips and advice. Also, to all the posters here that have highlighted thoughtful ways to discover more about the two schools.
The idea about attending a performance at each school and visiting a couple of sports matches was a really excellent idea and will be arranged - thank you for suggesting it.
We actually watched a play at another school where DS had been offered a place. Not because we particularly wanted to check out the school play, but simply because it coincided with an enforced overnight stay (the school meet the Head Master day started at 9.00a.m). Having gleaned a lot from watching the play, we will definitely arrange something similar at Eton and Harrow.
I started this thread to get more information about these two schools. My only motivation in asking the question was to get some information and ideas that were not generally available or in the public domain. I really had no idea at the time how much passion the question would engender.
We'll now visit again and ask all the questions you have so eloquently (in most cases) raised.
As a general update on where we currently are in the process, here's a bit of a summary and (sorry to say) another question.
Having previously sent us about ten House Master biographies, Eton Admissions Department asked us to pre-nominate one H.M from the ten (with no guarantee of meeting our pre-nominated H.M). We spent about a day going through each HM profile, their characteristics and interests (sport, music, academic), location of house, their tenure in the House (length of service/time left in House), eat in/out e.t.c. Having arrived at a very considered decision, about two weeks ago we pre-nominated our H.M.
Yesterday, the Admissions Department wrote, inviting us to meet two H.M's. Neither was our pre-nom (no issues about that), but one was not on the original list, so we do not have any biography to go from and therefore know nothing whatsoever about him 
.
Considering how clear, concise and helpful the Harrow Admissions Department has been, we are slightly puzzled that Eton (clearly an absolutely brilliant school too) seems to be a little less clear in it's communication. Can anyone with a D.S currently at Eton confirm if these initial exchanges continue in a similar vein were D.S to become a pupil? Or, if things are more straightforward when this ghastly process ends and we deal directly with the House Master?