For the record in the end I managed to fix this. After many false starts I went back to ground level and reinstalled git itself (it was not an easy process for various reasons) - anyway turns out we were running a broken install. With a fresh version everything appears to work.

Lesson being - if the error message is "not available or broken" and you know its available - consider the possibility of it actually being broken!!

Not in fact a Perl problem - apologies for the red herring