Just done some investigation. The centre pulley is that way up on my rear discharge deck. Large diameter at top and smaller at bottom. I've also had a look at some of the manuals on RS. They all seem to show the same. Large top, small bottom. (Behave yourselves now. )
Now someone has been the SD deck before. So did they fit the pulley upside down? If so why? Just looking by rack of eye, if the large diameter pulley is at the bottom, it will be very close to the tensioner bar. As in very, very close. I'll change it round tomorrow and see what transpires.