Willow or Wood?

Firstly thanks for all the congratulations on reaching 200 blips. I really didn't think I'd make it.

Secondly, I was working from home today and noticed a new visitor to the garden. Not the usual sparrow and wood pigeon today. I've checked my RSPB book and I think it's either a Willow Warbler or a Wood Warbler. But I may be wrong. Any ideas? All suggestions welcome as I'm stumped.