If you think you have spotted OPM, you should firstly report any sightings to the Forestry Commission via TreeAlert or by email to opm@forestrycommission.gov.uk. They can then keep track of the outbreak.
Secondly, please do not try to remove the caterpillars or nests yourself. They need to be removed by an Arborist who has had the correct training and has the necessary equipment require. The nests must also be disposed of properly to avoid the pest spreading.