Thanks Steve, Alfonso,

I spent a while at the business end which does have a threaded shaft allowing adjustment of the conical part that screws into the camera release. It helped with some lenses, not others. The moulded part which one holds seems to have some adjustment too, after loosening the small lock screws you can turn the chrome rod and this has a tiny effect. I think I'll just stick to a single release on the lens, the unpredictability of the double release is annoying.

Regards - Ross