I've downloaded MoreLocale and after setting up English UK (as a custom locale, it wasn't listed on the presets) I can see it has worked to a certain extent, although some numbers within contacts are still listed in the US format.
I've also noticed some of the settings are now Chinese, so I think I might just learn to live with the US format for now, the benefits of the ROM far outweigh the small niggles.
Thanks for your help again.