Tom, this may sound a random idea.. but I once had to contact Argos (the stores Customer Service Team) so I e-mailed them via the online form. A day or so later I recieved a missed call and a voicemail. When I looked at my phone the missed call was a Geographic number.
I never called them back, so sadly it didn't occur to me to keep the number.
Maybe worth trying though, and see if a: they call you back, and b: what number they call out on.
The link is:
http://www.argos.co.uk/static/StaticDisplay/includeName/ContactUs.htm