I picked up the BANDRIDGE 5 port automatic scart switch, and in short, I highly recommend it to anyone wanting a scart switch, it has 5 ports, is available in manual (£15ish) and automatic (£21) versions, the automatic is powered by a 12v 500ma transformer, has a manula override too, both have a seperate L+R audio out, and from what I have observed, no loss in quality, no loss in brightness( the image is acutally a tad brighter through this scart switcher) no noise etc, and its very cheap.
I haven't tested for interference but I never have more than one console on at a time.
Some images.
Rear view, its important there is enough space available between horizontally aligned scart sockets to allow for bigger plugs, the Bandridge has this, my Trilogy 2 doesn't