Finally got to the bottom of this one. I thought I had posted a fix already but I was mistaken, so apologies for taking a while to post up the fix.

Scoping the poorly corsa and good corsa on crank & camshaft signal I could see a definite difference on the cam timing.
I double checked the tools fitted the same into both engines, but on the poorly engine the crank locking tool was nearly such a good fit.
By measuring the position of No 1 cylinder on the good engine I was able to determine that the camshaft locking tools fitted at TDC.
So it was then a matter of slackening off the camshaft sprocket bolts on the poorly engine, setting up to TDC using a DTI gauge and protractor, locking the camshafts in position and tightening everything up.
Clear codes, road test and bingo - job done.