Thanks for the insight Alan, really helpful. I've certainly learnt something new!
Well I need to give a huge thank you to everyone on here, in particular JB
Turns out it was the constant pressure valve that had been left fitted to the front of the MC.
We stripped the MC last night and finally got it all bled up today and the problem has completely disappeared
As usual it wasn't without confusion or error, turns out we needed to ignore the F and R labels on the MC as they don't correspond to the front and rear brakes, in fact it was the opposite! Once we worked that out and swapped everything around correctly to match the Haynes Manual everything was golden.
Car is now ready to do nearly 3000 miles over Europe come Saturday