I scooted off pronto and thanks to some help from MR directing me to the appropriate spot I was soon watching a Western Orphean Warbler, a species I'd never expected to see in the UK. As I recall I think I've only seen one in Spain.
Look at the bill on that!
Western Orphean Warbler
I went back after the meeting and got some more views too. It dwarfed a Blackcap when it was beside one. The second photo shows the undertail, note the uniform pinky buff plumage which splits it from Eastern, nice.