Thanks for the quick response.
The serial number of the mower is 471179. The parts list gives the number as 004298 - it's the same number in the list you have on this forum for both old and new Morrisons.
The part I obtained from supplier was described as Genuine Masport Sprocket Assembly-10/23 Lh Thread Rm 004298, 032006.
The machine has a 1989 Briggs and Stratton 3hp engine so I assume it was built sometime after then.
Am away from home but when I get back will post photos and try to measure the pitch of the thread on the reel/cylinder shaft.
Thanks again.