Final resolution and update for reference. The Gates 6208MC went straight on and was easily adjusted without any issues. This tells me that the ERR2073 (which is 675mm) is not the same as the ETC7899 (which is 700mm) even though several sources suggest this is the case.
The 6208MC I fitted is 700 x the same as the ETC7899.
I also suspect the garage damaged the shorter ERR2073 / 675mm belt trying to force it on as they were sure it was correct.