Cheers for the comments. Had two new bobbins to try. None will thread. CC sent me new nuts as they said new bobbin was metric. Nuts they sent, according to the invoice, are 5/16" UNF full plain. These don't strike me as being me very metric! However they fit smashing on the old bobbin. Think I need to visit a local purveyor of metric nuts. As an aside do any of you think it's worth fitting Nyloc rather than standard nut and washer originally fitted?