A few years ago (maybe 6 or 7), I was trying to source an autopilot systems handbook for the Trident (dont ask - oh OK, I was taking a group of air cadets to fly a simulator we could actually afford for them). I wanted the handbook as I had previously flown the sim and remained continually 20nm behind it when the AP was on.
Unfortunately I dont have the contact I spoke to, but I did manage to get in touch with someone at Smiths (now a GE company) after a hell of a lot of phone calls. The result (after a bit of amusement that I was trying to trace this doc) was a full copy of the AP manual turning up in the post.
If the same people are still at Smiths then it is sure worth giving Smiths a call.