Ok, this should probably get moved to the IIS area of the forum.
After getting in touch with Aaron at
onesmarthost.co.uk, he advised me that I should add a rule in my rewriting module (ISAPI) to rewrite the mydomain.co.uk into www.mydomain.co.uk. This worked perfectly for me, and I no longer have a problem.
This technique isn’t working right now for mydomain.co.uk/services.html, but Aaron is working on a solution. If he finds it and I can get hold of it, I’ll put it up on the forums for all to see.
If anyone needs specialist MODx hosting, I can’t recommend Aaron enough. He’s been extremely helpful with everything I’ve asked.