If you have a functioning Sky dish there is no point installing the Ross dish. It won't give you any more channels, but it will give you a world of pain trying to align it properly if you've not done one before.
Get a quad LNB
like this one from Maplin and attach it to the existing Sky dish. Your unsubscribed FreesatFromSky box will be quite happy with it, as will your new Ross box, and you will have two spare terminals left over should you wish to add more boxes or a PVR at a later date.
---------- Post added at 23:22 ---------- Previous post was at 23:18 ----------
As to which channels you can get - basically the same as the official Freesat line up, plus a bit of dross around the fringes, and minus a few like Sky3, Fiver and FiveUS which are still encrypted (your Sky box will continue to receive them in Freesat mode so long as it has a card in it, but the Ross box will not).