So it has been a week of many tests on the green screen footage, including;
downloading a trial of Nuke, rotoscoping in Shake, tracking and exporting in Mocha Pro to name a few, plus different tests with Primatte and Keylight in Shake.
However, nothing produced acceptable results (surprised with Nuke).
But, on Sunday night I stumbled across a website explaining the virtues of a "KeyChew" Shake macro plus edge blurring using Saphire's Edge blur.
Found the macro (and a load of others) hidden inside Shake's content properties.
Gave it a quick go using a key from Keylight and a hold out matte produced from Mocha Pro and imported the shape data into a Shake rotoscope node...
This could be the way forward ... more tests to do!!