Thanks for the suggestion RugbyHukr, but it isn't likely, as they appear some 8-12 weeks apart so couldn't fertilize each other. I have been in touch with one of the holders of the National Collections of witchazels, and she replied yesterday to say that it is a characteristic of some grafted varieties that they produce
flowers the same colour as the stock
plant at a different time of year to the ones expected, both colours appearing on the graft, but nobody seems to know why or how it happens. It is a great relief to have my observations confirmed, even if no one knows the science behind it.