Wheel centre-cap, Rover 57mm.

[David]

It that the right part number David? Rimmers sell rover wheel centres but with a different part number

Yes. DTC000150 is the larger Rover badged one, 57mm, generally for V8 18" Vortex and 17" Star Spoke wheels, or the single Rover 75 V8 on 17" Viking wheels.
They also fit some non-V8 approved wheels such as Hairpin and Fork Spoke.
'Lesser vehicles' have the 45 or 54mm Rover version (25, 45 and 75 FWD cars).

David