I've phoned their technical department in the past with similar results, they emailed me spec sheets on different bikes I was working on and helped with a problem I had. Very helpful.
But a very useful thing from them is the spec book on all the old bikes with all the "new" old carbs they make. I'm sure they sent me one, but they also give them away from there stall at Stafford.
It's a shame people have problems buying carbs from them, they've always had the carb I wanted. But I had the same problem you mentioned in another post about tightening up the flange nuts, as soon as you do that the slide doesn't move. In the end I had to tighten it a bit, see where it was rubbing and with very fine polishing paper, take some off the slide. Then repeat until I could tighten the carb. I checked everything was flat on the head and the heat spacer, it all looked OK. I think they're made out of cheese 
email (option): horror@blueyonder.co.uk
I think the body/mounting castings are too weak...
Although the head face and the carb flange may be flat the fact a halite sealing gasket is used allows a sufficient amount of 'flex' to distort the carb body when the nuts are fully tightened...
If two gaskets and a Tufnol thermal spacer are used the position becomes worse...
If a flat surface is bolted to another flat surface no distortion is really possible so it seems to me the gaskets are the only thing that introduces a variant...Ian 
email (option): ian@wright52.plus.com
Yes I had the same thoughts Ian, the distortion that nips the slide happens at such extremely low nut torque values (Too low to even measure at a guess). It's heartening to here that it's not just me and that Horror has experienced the same problem and managed to get over it. However the process on a G3 might take me a month of Sundays unless I replicate the Tufnol block and gaskets on the bench/vice somehow.
I've sent my original carb back to Martyn who is more than willing to re-visit it and double check with the info from Amal.......I have just mailed them to see if it's possible to buy just a jet block which could just be the whole problem? (Or hole )
Incidentally, Martyn told me that the flange nuts should be done up to only 6 lbs torque. Ron
email (option): ronpier@talk21.com
['Incidentally, Martyn told me that the flange nuts should be done up to only 6 lbs torque']..
Unfortunately measuring that in position is more difficult than removing a Matchless carb...
...Ian
email (option): ian@wright52.plus.com
Yes you have to use an open end spanner and just say "click" when you think it's tight enough 
Ron
email (option): ronpier@talk21.com