Took the car back to Ritters in Burwood a second time and they kept the car overnight to drive it when cold with diagnostic equipment connected.
This time it registered a fault coming from the Manifold Absolute Pressure (MAP) sensor, which is bolted into the top of the manifold. Apparently, this sensor also has a temperature sensor in it as well.

They swapped this with a working second hand sensor to test for a couple of days worth of cold starts.

Drove the car home last night for 30minutes (ie. at full operating temperature) with no faults - drove really smoothly actually.
Drove the car to the station this morning for 10 minutes with NO loss of power/hesitation AT ALL!!!

Looks like this has worked a treat and I should now benefit from all the other mods that I've done along the way.

Fortunately, the wastegate shaft on my old turbo was quite worn and difficult to turn most of the time, so it appears that it was due for replacement anyway.

I hope this helps others with this annoying issue.

Regards Nigel