The easy answer is that the individuals at Shorelands came from Chester, which keeps proven individuals of said taxon. The vast majority of pygmy marmosets in the UK are non-subspecific hybrids, but there are a dozen or so collections which do hold pure Eastern. Although common on the continent, only a single collection in the UK - North Anston Butterfly House - holds the Western subspecies.
However the trickier answer is how to identify pure individuals of Eastern as opposed to hybrids, or indeed pure Western. For all intents and purposes the basic difference is that Eastern are pale, Western are much more richly coloured and hybrid individuals are somewhere in between. To a great extent telling the difference just comes down to practice; you pick up the "jizz" of the three different forms over time. Certainly the first time I visited North Anston and saw the marmosets held there I was instantly struck by just how "different" they were to the hybrids and Easterns I had seen before.
