Many thanks for the replies and links.
Goff, I'll drop Jason an email mate, his stuff does seem quite good, thanks
Simon from Mini Spares has also been in touch with me, he's kindly sending a new set from their most recent designs.
Doug, grinding Baulk Rings isn't a repair. The grooves within the Rings wear and so the grooves have less volume for clearing the oil from between them and the Cone and so become slow to allow for synchronisation, if at all. Usually too, because of the way they wear on the Taper of the Cone on the Gear, there's a step left in the small end of the Ring, which from my observations, when the Rings are ground and attempted to be re-used, only serves to catch the Molly coating on the cone and eventually lifts off parts of the coating. The 'Ears' on them also get battered about and so if the worn Ring will synchronise, the shift becomes notchy.
Also, the 'Ears' are a stressed part of the Ring in use and can crack, as yours is here and your 'mechanic' totally missed, but is pretty plain to see, even in your photo;-
Once they are worn, they are worn, fit for the bin only.