Hmm, bizarrely that has removed about a third of them but certainly not all. No idea why it's only picked those though.
I did git clean -n and it didn't look like the whole list. Then I did git clean -f and it deleted those files and confirmed it was barely a third of them. I'm more than slightly confused!
UPDATE
The remaining files are whole directories so that explains why they were left.
git clean -nd previews them and then git clean -fd removes them so all is now sorted.