As you rightly say, RLS is over-subscribed, so even if you were to move in to catchment, you would need to go to a Transfer Appeal, and no one can guarantee the outcome of that.
For late house moves, and where a child has already passed the Bucks 11+, the Bucks Admissions booklet states the following:
If you are providing evidence of a move that will happen between 14 December 2007 and 1 September 2008, any place we offer will depend on us being satisfied that you and your family are living in the home at the time that your application is processed or will be living in the home before 1 September 2008 and for at least six months after this date.We will ask you to provide evidence of this, including utility bills (contents or buildings insurance, a gas or electricity bill, a Council Tax bill or a TV licence) and evidence of ownership or tenancy.We will also want to see that you have sold or are selling your previous home. If we are not convinced that the move is valid,we will continue to ask for proof to show that you and your family actually live where you say you are.We may ask our legal team to investigate or demand that you provide legal proof of your address. We can refuse to consider a childâ€™s residence as being in
the catchment area if we have any doubt about the information provided.You must provide the information we need as proof.
I would only recommend moving into catchment for an undersubscribed GS, and the only schools that appear to fit the bill at present are Chesham High, John Hampden and Henry Floyd.
Moving into RLS catchment could be a very high risk strategy. If the GS your son has been allocated is "good enough", and you don't actually need to move, I would sit tight.