In reply to Runtothehills:
By choice, I avoid the hills at weekends. If I am out, weekends or holidays, it's usually somewhere quiet and lower level, or late afternoon / evening, when most people are off the hills anyway.
I expect that the option of being out late will diminish as I age, and take longer to cover the distance making routes undo-able. I'm quite lazy by nature, so apart from a handful of head torch walk-offs, I'm usually down before it's pitch black.
A few years back, I did a 4 month walk, without a vehicle, and often didn't know what day it was. Sometimes, during it, the number of people about on the hills, would suggest what day it might be. But as most of the walk route was new to me, the people seen on it were slightly less intrusive than the same number would have been back home, where I know the routes and conditions intimately.
The other thing which struck me was that the feeling of being crowded during it was generally not on the hills themselves, but at ( or near) the valley floors: car parks, and hostels. Even if there weren't that many people in a car park, the number of vehicles there gave off a sense of crowding, particularly where litter and inconsiderate parking were evident.