Possible ICTERINE WARBLER!
Extract from quick email to local birding groups with details:
I also saw what I could only describe as an Icterine Warbler but not 100% sure. From this field [small rough field SSW of gas tower frames at Boxmoor & E of wood at Lower Roughdown] you can get out at the top of the slope, nearest the wood, to the footpath that runs along the top of the bank of the A41. About half way along here at the edge of the wood a very yellow warbler popped up, more substantial than a Willow Warbler, very yellow, with a relatively long, thick orange bill, thick pale yellow eye stripe & a thick pale yellow eye ring that gave the bird an open-faced look with a big eye, with no dark markings on its face. In just a moment it darted along into a tree then I could not find it again, if flew much like a Reed Warbler or Blackcap, darting & in rush, diving into cover, unlike a Willow Warbler or Chiffchaff.
Grid reference for Google Maps etc. 51.739547,-0.486554