Mystery bird

Spotted a new visitor at the fat balls today.  I thought it was a robin even though our resident robin ignores the fat balls, but no red breast.  Husband thought it looked more like a blackcap, but without the black cap.  Our bird book suggested it was a garden warbler but also stated these birds migrate in the winter - found a good picture on the internet and it certainly looked similar.  The Bristol area might be milder than more northerly areas of the British Isles but even so... Anyone got any other suggestions what our mystery bird might be.

We live in a small town outside of Bristol with allotments and open farmland within 500m.  Sorry it was gone too quickly to get a picture.