Below are a couple of images of more typical olive-brown Grasshopper Warblers that were caught in the following days.
To try and progress matters I emailed Martin Garner to see if he had come across anything similar or to see if he could suggest any lines of enquiry. He agreed it was an interesting bird and requested some images for the Birding Frontiers website (link here) which focuses on identification issues and has a much better reach than my blog. Hopefully having some images published on that site may spark some suggestions but we still have to accept that we may never really know what it is in the absence of DNA testing. Whatever the outcome I still plan to submit it informally to the British Birds Rarities Committee (BBRC) for their consideration and for reference. If a bird like this can turn up once then it can turn up again.
One of the things I came across when researching via the net was a paper (link here) reviewing claims of Eastern Grasshopper Warblers in Britain (Harvey & Small 2007). This paper reported that the 4th primary was slightly emarginated in just over 50% of a sample of 20 of the eastern subspecies straminea and that this feather wasn't emarginated in any of the 12 nominate birds (naevia) examined. However one of the nominate birds I caught on the same day as the pale bird showed a slight emargination on the 4th primary.
I had only photographed this bird's wing because it had clearly gone through some kind of eccentric moult and had replaced a number of primaries in both wings. These new feathers also stood out as they appeared longer than they should be relative to the old feathers either side. I only noticed the slight emargination of the 4th primary when comparing photographs of various Grasshopper Warblers subsequently. Although the emargination on the 4th is only slight it is clearly there, especially when the feather is compared with the 5th, and would be recorded as such for the wing formula of this bird. The presence of a slight emargination of the 4th primary may not be the potential indicator of a bird worth further examination as that paper suggests and perhaps more importantly shows how much we still have to learn about Grasshopper Warblers.