I finally figured this one out so just thought I would share it with others.
Using the manual switching method worked for me. I ran wires 4, 5 and 6 into a
serial I/O switching unit (£60-ish from quasarelectronics.com). I was then able to send commands via the serial cable to fire the relays to open and close the curtains. Turned out to be very straight forward.
Now that I've got the basics done all I have to do is program the commands on my Linux server, (I think perl has some serial communication possibilities) and then schedule it all.
Wish I'd known that I was going to be installing the motorised curtains when I was renovating the house as I would have run some cat5 to the windows as well as everywhere else! Oh well, most of the wiring will be hidden by the curtains and other naturally positioned furniture so it's not too bad. Just waiting for the curtains to turn up now!
If you want additional details please do not hesitate to get in touch.
Thanks.