Right whiffey, these are my thoughts about your birdy. Not a Tree warbler, as the only one we have is the very rare Icterine Warbler, and not a Wood Warbler because as burnhal says you would notice the diagnostic yellow colouring. What is left are the other Leaf Warblers which have been designed almost identically - very helpful!!!
Personal traits can be good - the Chiffchaff dips the tail continually while darting about low bushes. The call (forget the song, wrong time of year) is a cheerful single note. Both the Whitethroat calls are a bad tempered clicking sound - so I think they can be eliminated, don't you?
My preference would be on the commonest of warblers - the Willow and they're all on the move south at the moment preparing for migration, although a few like it here and remain for the winter. Now this one is slimmer than the Chiffchaff, dips its tail much less but still flits about in the same way. The call though is different. It's quite a sad mournful amost a two note sound which it utters frequently.
Go on now, tell us it's something totally rare - and we are all wrong :-)