Ai!
I just noticed, I was in the wrong regex rename mode...
tsk tsk tsk...
Pretty embarrassing.
From what I remember, a lllong time ago, there was something with the maximum groups, or something and one could only use upto \9 ...(?)
Hence when nothing happed when trying \20 I wrongly assumed I hit on this 'limitation', but did not realize I was using the wrong mode.