I spotted the dunnock a couple of weeks ago and was hoping to get a better photo, but I haven’t seen it since. It is much more speckled than a sparrow, with a darker speckled back. It has a straight-out tail, rather than sticking up like a wren. It has a thin beak, not chubby like a sparrow (though you can’t see the beak in this picture).
It has wren-like behaviour, frequenting the lower boughs and shrubs, hunting for insects to eat. This one was perched on the watermeadow fence behind the alders.
Dunnock update 05/01/24 – I have replaced the featured image, because I have now realised that the dunnock is a regular visitor to our bird feeder, which enabled me to get a better photo. It hops around the bottom of the feeder and bushes like a wren, but here it was perched up in a small tree. You can clearly see its thin dark beak, which has helped me separate it from sparrows.