I believe (although am open to offers
) that this year, for the first time, no children (unless they had siblings at the school) were allocated RLS if they lived out of catchment i.e. MK in the first round of allocations, some may have got lucky later in the year (May/June) but it was an unknown quantity waiting.
I think our village in Bucks is the furthest away from RLS (and the nearest to MK) so if they extended the catchment/distance from the RLS any further it would open up the school to lots of eligible MK kids.
I know of quite a few MK children who passed the 11+ who were not allocated a place at RLS but who later took up a place at one of the three Aylesbury grammars and who are bussed in and out of Aylesbury via the A421.
So, if you are adamant you want your son to attend RLS I would make sure you live in Bucks and within say 8-9 miles of the school, if you are happy for them to attend the equally good (or better) Aylesbury grammars and want to live in MK then I would live within walking distance of the A421 to easily catch the school bus.
Good luck, I am sure you are aware that the Kent 11+ is different from the Bucks one, but plenty of help on this site which ever direction you turn.