They are often reared by enthusiasts, which no doubts explains its presence here.
This is a female; she has smaller antennae than the male, which is more brightly coloured and which has swept back, falcate, forewings. Both sexes have long swallowtail streamers, but these have probably been pecked by birds or lost in general wear and tear. The female lays 250-300 eggs. The caterpillars accept a wide range of foodplants including Rhodendron, Hawthorn, Oak, Lilac and cherry.
Related LinkSaturniidae of the Western Palaearctic by A.R. Pittaway.