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Second thoughts, think ophrys has this one!!!
I have just looked in my Hamlyn Guide to wildflowers and Ballota nigra is in the 'red' flower section. I would have seen it as a lilac or blue. When I go back to site today I am going to have a sniff at it as my book says 'whole plant rank smelling' so that should be easy to confirm!
I'll take some more pics of it today including close ups of leaves hopefully (photography skills permitting!)
Lat/Lng: 51.523291452272, -0.02951662579494
OS grid ref: TQ368823