Hi Gruditch,
In answer to your question: yes they are really!!
Gymnosoma rotundatum is known from about 10 - 12 sites in the south east, and where it does occur it can be fairly numerous. However, it is currently thought to be restricted to Surrey and Sussex, although Hampshire and Kent
do have areas of suitable habitat, and there have been one or two records (although not as far as I am aware from Farley Mount).
In short, you may have stumbled across a fairly significant find!!!
The chances of mis-identification are low as it is a very distinctive insect.
I shall do some research and follow this up and let you know.
Cracking find..!
All the best,
Felix.