Mountfield Emperor MP83309 Self Drive Problem?

Hi, my Mountfield lawnmower is giving me problems at the moment. The drive either doesn't work or drives all the time. I had it dismantled at the weekend and fitted a new belt and wirebrushed both pulleys to remove rust/dirt from the seat of the belt. Howeverl I noted that a side plate covers the belt drive for the most part but a small section of the drive belt at the power shaft pully end is not covered by this plate and is open to the elements which may be allowing grass cuttings to access the area. Should this section be covered by a second small plate to keep the belf free of grass or other debris? After my work the machine was doing well and suddenly the clutch handle went slack. I thought the jockey had jumped off the belt but it hadn't. The belt seems to have removed itself from the roller pully. I plan to investigate this but suspect contamination by longish cut grass has forced the belt off the pully.
Can anyone shed any light on the matter?