It soon became apparent that this was no dragonfly and a discussion ensued as to whether it was a Hornet or not. I thought "not" as even with it zipping around above our heads, it did not seem yellow enough. It was big enough though, which puzzled me. Not an odo, probably not a Hornet.
Then it landed on me. Definitely not a Hornet then, but, my, look at the size of that ovipositor. At...least...I...hope...that's...an...ovipositor.
Fortunately, it then flew off again, but I was able to snap a pic when it landed on a vehicle.

As a Google image search might tell you, if you happened to enter the words "large yellow insect with ovipositor uk", this is a female Greater Wood Wasp or Horntail. She's 4cm long and is actually a sawfly. Despite being quite harmless, she does makes you glad you're not a tree. After terrorising a few more humans, Mrs Horntail was last seen in combat with a rather brave and normal-sized wasp and we returned to our everyday activities, feeling a little less removed from Mother Nature.