Hi John
Just wondered if you progressed this any more, I too am now looking to build a setup in a shed and too wondered about doing a Merstham to Redhill layout. Some of the old photos are amazing and I had forgotten how much railway usage was going on in the Redhill area. I was hoping to find more on the Redhill sidings but with not much joy other than the photos in this link.
I too was at Redstone and Springvale, at the same time as yourself!