Colchester appear to just have the one (based on what I saw and ISIS) so at the moment this is the only duiker on show in the UK. There have, however, been strong rumours around (including on these very boards) of at least one UK zoo getting Natal Red Duiker very soon, which I would love to see in the UK. The species is booming (by duiker standards) in Europe, so I think it's only a matter of time.
I knew they were uncommon but didn't realise I was looking at the only Duiker in the Uk when I saw it. It lives in an 'Edge of Africa' enclosure with Stanley cranes. These are somewhat better(more natural) enclosures than in the older part of the zoo.
I think this animal has been at Colchester for a number of years and is actually the survivor of a small group which have now dwindled to this one animal, unless someone can correct me and this is in fact a new arrival.