Hi Graham,
It is doesn't look like either a Wiilow Warbler or Chiffchaff. If anything, it looks like a Garden Warbler - whose main distinguishing feature is that has no distinguishing features at all!
It could possibly be a Nightingale, but if you have a look at my website, I have a number of Nightingale photos which I took earlier this year, and you may be able to make a comparison.
http://www.wildlifeindigital.co.uk
cheers
Ian