Mountfield sp185 problem

[tomo658]

I've had a mountfield sp185 for 3 years and I've never had a problem up until now. I've no experience in trying to repair lawnmowers so I'm hoping someone can help so I can give it a go.
The back right wheel is the side with the height adjuster on. The back left wheel keeps giving way and moving out of position so it isn't flat to the ground like the other 3 wheels.
Has this happened to anyone before? Thanks in advance

 

[Burnerman]

Remove the back left wheel by removing the centre bolt or nut, then remove the small drive gear ( circlip pliers required).
Look for the peg that drives this gear and observe its orientation unless its a simple round peg, also look out for any washers that act as spacing shims.
You’ll now see the plastic backplate which has somehow disengaged itself from the rod that comes from the right hand adjuster and the fault could either be a split in the backplate itself or a large washer has come off the adjusting rod.
Give us a pic if you get that far?
John