Thanks, John, for the appraisal.
Yes, it would be nice if the stopper were ebony and it would be even nicer if I had a lump of ebony big enough, but all that I've got is 2 small strips, about 10mm thick and 70mm wide. I had thought of making the stopper out of the same wood as the filler in the joint line, but didn't want to get into using any of the browner woods that I've got. Once the ebony ring was fitted I realised that black was the perfect choice. I've got a couple of blackwood blocks about 100mm square by 70mm thick, but I'm loathed to cut them up just for a stopper, so it looks like I'll have to settle for the myrtle.
Thanks for the suggestion, anyway....Les