Might not need an amp that switches RGB Scart, depends on your setup.
I'm guessing that you haven't got two scarts on the DVD? If you did, you could wire it up as follows:
Sky -> DVD Scart In
DVD Scart Out -> TV AV1.
That way, when the DVD is taken out of standby then the DVD picture switches onto the screen, and when it is off, the Sky picture is on screen.
An alternative approach is:
Sky TV Out -> TV AV1
DVD Out -> Sky Aux
When Sky is off, the DVD signal passes through to the TV. This doesn't need two Scarts on your DVD, but you may have a video plugged into the Aux Scart.(Less of a problem if you are using Sky+ though)
Your Video can only output Composite, so plug this into TV AV2 and use either co-ax from the Sky box or (If Video has two scarts) loop through in front of the Sky TV Out....
So the point of all this? Well if you can auto switch using loop through you only need an optical switch. Keene have one for £60 that can switch up to 4 inputs to 2 outputs.
See
http://www.keene.co.uk/cgi-bin/codesearch.pl?BTZ42
Hope this gives you a viable solution without spending money on an amp, seeing as you have paid for the decoder in the TV
Kind Regards,
James