I was unable to get any photographs of the bird but Barry Wright from Kent had visited the day before and has kindly allowed me to use his photos to illustrate this blog post.
Any wintering Lesser Whitethroat in the UK is very likely to be of eastern origin but we went with an open mind to just report things as we saw them. The bird is quite noticably pale with a pale brown mantle and lower nape. Structurely it was similar to 'curruca', not looking small and 'Desert Warbler-like' that the race 'halimodendri' does. It also appeared quite long-tailed/shortish winged (primary projection no more that 50% which can just be seen in the pictures below) and the bill was also relatively strong and long. Although we didn't see the pattern of the outermost tail feathers we were lucky enough to hear it call a couple of times - a clear 'tack' which again does not match 'halimodendri'. The overall paleness, pale brown mantle tones, structure, primary projection, bill and call all point towards it being a 'blythi', known as Siberian Lesser Whitethroat.
Stop press - @13/04/18, dna analysis has confirmed this bird as 'blythi', as per Martin Collinson
presumed Siberian Lesser Whitethroat, Sylvia (curruca) blythi, pics courtesy of Barry Wright