I have made a bit more progress.
Say that you were using the old pscan.eu version of pscanloader.
When you run it, it will download version 0420 from the old pscan.eu server.
Version 0420 will then download the new pscanloader from pscan.uk and relaunches it.
Unfortunately version 0420 continues to run in memory (but not visible), I found it in task manager.
So, when the new pscanloader attempts to download version 0421 and save it, it can't because version 0420 is still running in memory.
The solution.
Either find pscanwinapp in the task manager and kill it
or
Restart the computer.
Then run pscanloader again.
This time it will successfully upgrade to 0421, and, as the old pscanloader is now gone you should not experience this again.