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I think they were discontinued a few years back, I was looking for some a while ago but came up short, I have the white rose set as well as regular star so was always looking out for centre cars. regularly checking eBay could be the best bet.
You could try East Kent Models in Whitstable, they have tons of Hornby spares, so even if they don't have complete models, they'll probably have the bits so you can build them
If the spares list they sent me is anything to go by, they don't have anywhere near enough spares to build them, but that doesn't mean they won't help if they can.