Alistair,
It is never that simple ….
According to the Spares Catalogue, there are two possibilities –
Pt no 88264, 22 diameter thread x 1.5 mm pitch
Pt no 604016, 20 dia. x 1.5 pitch but with 2 tapped holes, 4 dia x .75 pitch.
Unfortunately the book does not state when they were fitted and/or when they changed. But, as yours is a late car, I would guess it wll be the second (20mm) option but that is a pure guess.
If you can measure the thread on the column, with a 2mm difference, it should be fairly obvious to determine which you require.
B…..