I'm posting this to hopefully help anyone looking for this information in future (via the search facility).
I've been looking for a replacement drive belt for my wheel autopilot. Others on the forum helpfully pointed me in the direction of Beeline Engineering as a supplier of these belts and quoted a replacement part number. My thanks to those individuals for their help but I now want to add some further clarifying information for the benefit of others in the future.
The ST 4000 Wheel Autopilot comes in two versions - the Mk I and the Mk II.
The Mk I version has a black wheel drive ring. The Raymarine part number is D169 but this appears to be no longer available. The replacement belt is a SYNCHROFLEX AT5/750 and is 9mm wide. Beeline stock this belt and will cut it to the required width so make sure you tell them you want a 9mm belt.
The Mk II version has a grey wheel drive ring. The Raymarine part number is A18083 but this appears to be no longer available. The replacement belt is a Gates PowerGrip 506XL050.
Both belts are available from Beeline Engineering who I have found to be incredibly helpful.
They can be contacted on 01908 222999 or at http://www.beeline.co.uk.
I have no connection with Beeline - just a satisfied customer.
Hope this is of use to others either now or in the future.