I am sure I found them somewhere when I was making a table but in the end I chose a different system from Hafele with what is effectively 2 drawer sliders with a cable mechanism that equalizes the sides of the table. The reason I changed to this option is because it was far smaller and lower in profile, plus it runs lovely and smooth on ball bearings. Also less likely to be affected by climate and moisture.
Thanks Ollie, that would be a good solution except this is a really trad-style table, so I don't think the client will like it. Guess I'll have to order from the States....