How do you change the deck belt on a mountfield 1228 sit on mower?

i have purchased a new belt for my mountfield 1228 sit on mower, however i can't seem to get the old damaged / stretched belt off to get the new one on. It has two metal guides welded to the chassis at one end and what appears to be a fixed metal guide at the other ,.... neither of which appears to move .. Help..
thanks