Thanks to IR a close encounter with a Marsh Warbler. I have spent the last two weeks listening for this 'King of Mimicry' without success.
The combination of dull olive grey tones, yellow feet, pale primary tips and evenly spaced primaries are good pointers to a Marsh Warbler. However Janet and Ian have done the Maths and the Walinder Formula also agree. Singing birds can be much easier than the maths.
Now it seems he is not alone. Female trapped and ringed with brood patch so I'll remove post for a while!
Just in case you're interested:
Length of bill to Skull (A)
Width of Tarsus (B)
Width of Bill (C)
Less than 8 = Marsh
8.5 and above = Reed