Rear disk brakes

[NeilW]

Back again with further woe's!
Just fitting my rear disk brake assembly (Sierra) and noticed that I don't have the splash cover/backing plate. Is this a critical requirement?
Currently I have a load of free play in the driveshaft through the upright but I think this is due to not tightening the 36mm lock nut yet...still waiting socket from the bay.
Reading up about the backing plate there are a few comments saying people run without them but I am not sure for Iva. As usual they are like rocky horse **** to get hold of.