Is your discovery an early 300tdi by any chance (I'm guessing it is due to having a 4 spoke steering wheel rather than a 2 spoke)?
defenders have either 36 or 48 spline steering wheels.
Being a 4 spoke steering wheel I think you will find that your disco has a 29 spline shaft (take the steering wheel off and count the number of splines to confirm this). If I'm correct you will need a steering wheel boss for a rover 214 up to 1995. If you have a later disco you may well need to just get a kit for a 11/16" shaft with a 36 spline boss.