You must have rearward facing mirrors as follows;
In all cases an offside (drivers side) external mirror is required.
An internal rearward facing mirror is also required unless there is no rearward visibility (such as a solid bulkhead)
If there is a rearward obstruction such as a solid bulkhead then a nearside mirror (passenger side) will be required
In the case of 3 wheeled vehicles with no bodywork then an offside and nearside mirror is required.