I may be too late to reply to the OP's request - if you've sent their local authority request in - but if not, I'm thinking you may live locally to me and your children may attend the same primary school as mine. In which case, if you take a look on the website at where Y6s head off to each year, there's not a lot in it between St. Mark's and SRRCC. I feel your pain making the tough choice, however, we found talking to the head of our children's primary really helpful. She was very fair about - and a cheerleader - for both schools (as well as St. Paul's and Gumley House, another two excellent schools, where just a few of the cohort head off to each year) - but her advice helped us come down on the side of SRRCC. We're glad she did. Our children are very happy there but yes, the school does have its faults.
On the whole, discipline is very good, bullying is not a problem and the teaching - as others have said - is excellent. Where the school does fall down is that it's not quite strict enough to allow for the awful truth that there are some terribly entitled parents who seem to think the uniform rules - as well as taking in contraband items such as headphones, or wearing make-up - do not apply to their free-thinking, darling children. The school issues negative points and detentions for the plethora of skirt rollers and mickey takers, but if the parents aren't acting on these and ensuing their children stick to the school's policies there's only so much the school can do. It's not pleasant for the students who do abide by the rules however.
I did read on another SRRCC thread about a child who was getting harassed by other girls for not rolling her skirt up, and this happens to my daughter (resolutely unrolled skirt, just above the knee) on pretty much a weekly basis. It exhausts her. I'd love for the school to address this aspect of behaviour as it tips it all into something truly uncomfortable for those students who do conform. I wonder if the school management have got their collective heads in the sand here?
I have friends with children at St. Mark's and I truly believe the major difference between the two is demographics and how the families (many of whom are first generation immigrants) value education and respect the rules/boundaries the school sets. It's great you picked up on how happy the students are at SRRCC, however, seeing some of our children's more feral friends, we do sigh sometimes and wish the school would lose a little of the 'love' in their motto and help the students understand that in the real world of work that will be upon all the students before long, there are rules and standards; that the world won't think them as special as they and their parents believe themselves to be; there are repercussions to be had from rule bending. I think this is something St. Mark's excel at.
I could go on. In short, however, if you send your children to SRRCC with the message that YOU won't put up with micro skirts, etc, they will thrive. It's mostly a terrific school. Outstanding in fact. OFSTED were not wrong on this one.
From what I hear of St. Mark's, it has its fair share of problems too - and nope, I wouldn't have wanted my children leaving an after school club at 4 pm in December and travelling through Hounslow (I grew up in Hounslow and I definitely wasn't offended by your comment) - and although I think they've improved their game on the pastoral side, the emphasis does appear to be on getting their sixth formers into Oxbridge than ensuring the whole school is safe for all. That's just my opinion, however, and there's nothing wrong with ambition. I only really know about the school through hearsay and chatter but it does sound wonderful too, albeit very different to SRRCC.
Friends with children at St. Mark's seem just as happy those at SRRCC, so you won't go far wrong with either, I think. Talk to your primary head. If it's who I think it is, you'll be well supported to make the right decision for YOUR children. If, IF, you could stretch to putting St. Paul's down on your list and found a way to get your children to Sunbury every day, it sounds like you'd be ecstatic about it. They seem to be getting everything right INCLUDING their sport provision. It might be nuts to rule it out when it is so good right now.
One final thing to add, however, is: it's anticipated that all good state schools in Richmond will be affected by the numbers who might have considered private school for their children but who the added VAT on school fees will be just too much. SRRCC is always over subscribed but it might be a bit trickier from here on out to gain a place. St. Paul's will probably suffer too, but St. Mark's less so. There's not much any of us can do about this but if you get St. Mark's over SRRCC because it all goes crazy next year, your children will still be in a wonderful place. God bless them on their journey, and you.