Hi tubmum,
Sorry to reply late, been trying to find the time to write you a decent reply.
Firstly, how old is your DS and when are you applying for? I'm assuming Sept 2013 and either Nursery or Reception?
There are a couple of important questions you need to consider - firstly, do you prefer co-ed or single sex? I guess this is just personal preference and it's probably too young to tell which would suit your DS best.
Secondly, do you want a school that goes up to the end of Year 6 and prepares children for 11+ entry to private and state schools, or do you want one that goes up to the end of Year 8 and therefore prepares for Common Entrance?
The big point is that the prep schools that prepare CE will tie you into a contract that will not let you sit you child for local private schools at 11+. You would only be able to try for state schools or very much out of area private schools at that point, and the school will not be preparing the children for 11+.
So in Northwood you have:
St John's
St Martin's
Northwood Prep
which all prepare CE. I know a lot of people with sons at all these schools, so have a fair amount of info on them.
In Pinner you have Reddiford and Buckingham College Prep. These both go up to 11 and are a lot smaller. We have our DS in Reddiford, he joined in Year 1 (we came back from living abroad). We have been happy with the school and DS is happy there. It is co-ed and suits him as he is not a sporty/boisterous type (not that he's particularly academic either). He gets on well with the boys and the girls and has made nice friends. Acdemic standards are high. I can tell you a lot more if you like!
I hear very nice things about BCP too!
Harrow schools I know less about - there's Orley Farm, Alhpa Prep and Quainton Hall. We have quite a few kids at Reddiford who have transferred over from Quainton - I don't know why the parents were unhappy with it. I can get more info if you like. Orley is supposed to be quite sporty - can anyone confirm?
Don't rule out pre-preps either - The Hall in Northwood is very small but lovely with amazing academic standards. I don't know Roxeth Mead at all. Innellan House closed down a while back.
Also, do you have any other children? If for example you have a girl as well, it might affect which schools you apply to (you might want to apply to co-ed, or apply to a school close to other girls' schools).
I hope I have helped a little, feel free to ask any other questions you like!