Liam Hutson:
I think that this sort of upgrade is usually a shop-made effort calling for some straight forward turning and dividing. GHT's MEs Workshop Manual may give you some ideas about how to go about it. You would need to identify the leadscrew pitch of course and decide whether to go for radial or diametral marking ie whether a given depth of cut is off the radius or diameter of the work. From your photos it looks as though one problem would be the location of a witness mark.
I see from the link to Tony's website that on some of these lathes "cross and top slide were fitted with, for the time, generously dimensioned micrometer dials" and it would be helpful to know how the manufacturers approached the problem. He can be very helpful but does say that his email load is very heavy.
I can't suggest a specific source of an existing item and, apart from the cost of such things bought as spares, there might be some reluctance to sell them for use on a "foreigner".