Just had the same thing on my 45s508d.. I removed the two screws holding the top.. then put my arm down to the bottom to find the motor pulley... put the belt on that and then fitted it to the drum pulley....very hard to do , I got a very sore arm after fitting it..I found a lot of old belt on the bottom pulley which I had to remove by feel only and a pocket knife.. as an ex engineer I found the the worst belt I have ever replaced...As I could not find any instructions of how to do this it was trial and error... Good Luck my old belt was very poor quality only been on 3 years..
Barry Hardy
January 2008