Dont know how regular they are in Thetford Forest these days-there was a discussion a while back suggesting most if not all reports were mis identified Marsh Tit.
Got me thinking though, because there was a 'Marsh Tit' around the feeders in the car park which had a rather buzzy chay chay call and a song which was a bit different to what I remember from my local Marsh Tits (and I have not heard them singing for a few years now). Twenty five years ago, Willows were regular in Stevenage but rapidly declined and I have not heard a singing one for maybe twenty years so am a bit rusty.
I've checked a few sources and compared songs and the bottom line is I should have been paying more attention.
So I've ticked Willow Tit for the year.