Did you replace the starter or just buy it....
Spinning starter means one thing...no tooth engagement between starter gear and ring gear. This could be caused by; faulty starter drive, broken or worn teeth on gears, mismatched starter/ring gear. Probably can rule out that last one. That leaves you with those other unpleasant possibilities.
Be sure that your electric connections are clean, tight. Double check your engine ground connection.
Replacing a starter on a rover v8 is bad business, so try the connections first.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
All my troubles are Rover