OK, Update and further Q (which I think I know the answer to, but thought I'd check....
So - the G- clamp idea worked a treat.
If you have a similar problem ,take 1 x 3" G- clamp, and one of those mid flanged pins from the flatpack furniture (if your boss doesn't notice one can be acquired temporarily from under a shelf in a kitchen cupboard!
Any how, I hend the catch in with that, and then applied big hammer to the now seized engine end. this slowly but surely knocked the sheath free of the helm unit.
This is where It's all gone the way of the pear..... I then turned the wheel to get the inner bit out. The free end went in, but nothing came out the cable entry end. To the tune of about 2.5".
I think it's safe to assume I'm buying a new helm tomorrow....