If you are an LDMCC member, try the club's spares scheme - but, beware, there were 3 versions of the top head bearing on the Mark series machines. The original bearing was in the form of a thrust race with 2 identical halves (numbered 2016). The next version was a cup and cone type with the fork top plate bearing directly on the thread of the steering head stem. The later version had a spigot for the top plate to bear on, and the hole in the top plate was opened up accordingly.
The early bearings (2016) are no longer available from bearing stockists, but the Club was having the later type re-manufactured.
Regards,
Eddie.