That was one of the posibilities I came up with before posting but I could not detect the yellow in the breast of the images. This is what made me discount them. This bird was not in song so I could not use that as a guide. By the way, I am using Collins Bird Guide as my reference, pictures page 307.
Admit that this is a very pale "washed out" bird Peter, but Joe is correct that it is a Chiffchaff. But they are VERY variable in terms of the amount of yellow in their plumage - I see birds which vary from drab pale grey and brown (like your bird) to individuals which are lemon-yellow on breast and flanks and greenish-yellow on rump and upperparts.
The only thing this could reasonably be is Chiffchaff or Willow Warbler and in the absence of song or call the dark legs make it Chiff which is confirmed by the short primary projection (shown well in your second photo).
Thanks Colin and Joe,
Its not as easy as you might think but I thank you for your advice. Chiffchaff it is then.