With thanks to Tim V, Nearholmer and bertiedog for your inputs and ideas.
The 'Mystery' loco original posting is now solved; pretty obvious really. As already suggested it's a (much modified) K's, the Mallard Models plate is a red herring, it's a scrap piece of a different kit etch reused as a motor mount!
As for the running qualities, the main problem is revealed on fitting the loco body to the chassis, the drive shaft runs between the underneath of the cab floor and the chassis, the clearance between is tight, as the shaft rotates it just touches the cab floor which acts as a sounding board. The cab is nicely detailed and I'm not keen to raise the floor, I may be able to also tweak the wire shaft enough to help matters but it's doubtful.
So I think it's a nicely thought out and detailed kit conversion that was not completely carried through to iron out the snags, a major example being the motor flywheel which was so far out of round and balance that it had the effect of a pager buzzer, easily cured though with a coarse file gently held on it whilst running! { )
Oh the joys of other peoples unfinished projects, nice one though it is!