Thanks Jim

I guess the rubber makes them less brittle then.

I've only ever had one plastic bobbin break on me so far - and that was one of my old Elna ones while I was winding it - so fingers crossed I won't see one again!  In fact these pre-wounds look exactly like my Elna Diva ones with the flat tops, and in fact those Elna bobbins also work in the Janome, and the Janome ones work in the Elna.

These are the Elna ones I'm talking about - http://www.ebay.co.uk/itm/SWISS-ELNA-SEWING-MACHINE-BOBBINS-x5-1000-2000-3000-5000-6000-8000-500-Club-etc/281598332794?_trksid=p2047675