Confusion: Scart Switching, RGB, red-saturation, etc

I hope someone here can shed some light on the problem I'm getting.
I've looked through and seen similar problems, but nothing quite the
same.

Set-up: Sony Widescreen TV with one RGB scart. Freeview box (Humax
8000T). DVD player.

Problem: Would like an RGB signal from both inputs.

Solution: Get some kind of Scart switcher.

1=2E Tried the manual one from Argos (the single box without a lead
connected). Very bad results. Widescreen switching screwed up, sound
interference.

2=2E Tried an automated one in Curry's today (Ferguson, =A315, F100 or
some-such). Claims to be fully RGB and 21-pin compliant. Plug it in.
The DVD player picture comes through fine. The box picks it up whenever
it is pluged in. The Humax signal is picked up and looks sharp, BUT the
picture is saturated with a red hue. Apart from little blue-green
patches in the corner.

I tried a variety of the inputs to the scart-switcher but all were fine
with DVD, red with Freeview.

(Interestingly, the Freeview picture direct to the TV is fine, but
there are tiny blue/green patches in the corner if I look closely).

I am mystified as to what is going on. Why should be freeview picture
be OK going direct, but red when throught the switcher?
Any ideas?

Thanks

Toby