Warbler.

Tricky little birds are Warblers, I can cope with identifying most LBJs (Little Brown Jobs) but warblers I find difficult.  I am tentatively saying this is a Juvenile Chiffchaff ( Phylloscopus collybita ).  I noticed a bird moving about in the Amalachier and the the way of moving wasn't the same as the usual species.  So I grabbed the camera and took some shots without even checking the settings.  Thankfully the shots came out reasonably.

Chiffchaffs are a species which are doing quite well.  The breeding range has increased by 18% since the 60s.  Perhaps a sign of climate change, the overwintering population is up by 84% since the 80s.  The first documented case of one spending the winter in UK was in the early 60s.