Peter,
The deformity you photographed may have been caused by a gall. There is something similar depicted in Margaret Redfern's and Peter Shirley's 'British Plant Galls'. It is not my subject, however, and I suggest you get in touch with the County Recorder Jerry Bowdrey. His address and telephone number are in the 2009/10 Essex Naturalist or are available through the website.
Graham Smith