The Smooth Stick-insect (Clitarchus hookeri) was the next Stick-insect species found in Britain. I was found in 1949, also in Tresco Abbey Gardens (where the Prickly Stick-insect was first found.)
Stick-insects are slow moving and although they breed fast, they stay in the same area and so, in the 52 years the insects have been found in South-West England, they haven't spread far.