Ok Guys I've now received an official response from the CAA:
There is legally no formal requirement for the differences training to convert on to the Kolb Twinstar Mk III to be conducted by a qualified instructor because you have already flown aeroplanes with three axis controls.
However, looking at our records I do note that your experience of aeroplanes with three axis controls appears to be in types like the Cessna 152. While the Kolb Twinstar is also a three axis control machine, its configuration is rather different to types like the C152, not least that it has a pusher engine and tailwheel. You might wish to reflect on whether being checked out in the type by a qualified instructor with experience of the type would actually be worthwhile.
Of course I can't say whether this would stand if going straight from GA to 3 axis as to fly a microlight on a SEP rating still requires 'Microlight' differences traing under the regulations. BUT having done this training on a flex and having previous experience on 3 axis it would seem legal to continue my training on the Kolb without the need for an official instructor.
Hope this helps those in a similar position in the future
