I did the London Walks day trip to the Cotswolds and really enjoyed it -- it was a lovely day out, and something I couldn't have really done otherwise on my own with a day from London.
However, I almost didn't get to go because so many people showed up to go on the walk!! I had arrived at the meeting station (it was Paddington, I don't suppose it's changed) early, but kept wandering around because I thought there was no point in going to the meeting point before time. Boy was I wrong. If it hadn't been for a VERY kind Indian girl who had just moved to London to study, who GAVE ME HER PLACE, I wouldn't have gotten to go (I thanked her profusely, sent her an email later to reinforce how much I appreciate it, and then later sent her a ticket for a social media workshop with a friend of mine who's the chief digital officer at the Met. It was no small thing that she gave up her plans that day so I could enjoy the Cotswold sunshine!!).
So if the advice is still to go find the fellow in the red hat by the such and such info point, you would not be crazy to go 1/2 an hour early to make sure you get a spot.
Somebody (maybe even a couple of people) has posted here on RS Forum about their experience with Secret Cottage tours, if I remember correctly. You could do a search on the site. I remember they were very enthusiastic about their experience.
Anyway, for the London Walks tour, I felt it was well worth the money I paid. They took care of the transport and designing the tour, all I had to do was follow along and enjoy the walk and the sunshine, and the fellow travelers I met!