This is getting good!
After checking, looks like i was wrong on the number of primaries beyond the tertials on Booted. They can have seven. Moral: always check more than one picture!
However, i had a feeling that Booted Warbler can show almost a small notch to the tail, whereas the mystery bird clearly has a slightly rounded tail. See these two Booteds (they are autumn birds but i don't think it should make a difference for this) which seem to support this.
warblers have taken up many trees worth of ID papers and correspondance in the journals. They can be a very tricky group! I'm going to have to resort to the unscientific view that it just looks too long for a Booted, with a large bill that has a completely pale lower mandible. Booted usually has a dark tip to the lower Check out:
Athough these ones do have a completely pale lower mandible...
However, i'm yet to find one with striking pale fringes to the secondaries and tertials, and they always seem to look like this:
with the closest match being this bird:
This is a very interesting bird! Still think thats it an Upcher's based on a number of factors that have been mentioned throughout this thread. I think that while there are individual features that may be shown by Booted as well, they don't all add up to make the complete package. I've been wrong before though...