I got this sorted in the end. I was provided with a copy of the v50.2 daten files, which had the necessary flash data files. The end result was the same though, the ECU would refuse to be flashed in WinKFP.

In desperation, I went to see my auto electrician mate, who has a GT1 and Progman v37. Updating the module using his setup had the desired effect.

I think I'll avoid anything more complex than coding from this point on...