Check this out and PM me if you’re interested. https://forum.beoworld.org/forums/topic/adapting-ortofon-om-series-for-sp-turntables-small-batch-coming-soon/

The SP12 does have an elliptical diamond - the others are spherical. I note there is a firm selling newly restored styli with a choice of tips though have not used him - audio-origin is the name. Seems to be retipping original cartridges. I have a bespoke SP2 cartridge with an elliptical diamond supplied by Frede at Classical Audio - fantastic cartridge.

Thank you very much Peter and Territorial_ for responding. I have seen audio-origin on ebay in the last day or so and am thinking of giving them a try. I have an SP10 and an SP14, both of which appear to be still have perfectly smooth tips under microscope but I guess the cantilevers may have taken some wear over their nearly 50 years. The advertised SP10/12/14 elliptical is a pricey matter but I'm hoping it will be worth the expenditure quoted at £269.