Black cap like stonechat but mustache more like whinchat.
No russet front. No eye stripe.
No interactions present.
One of the main features for Whinchat is that in all plumages it has a broad, pale supercilium (the stripe above the eye). This bird's lack of that immediately suggests it is something different...in this case, Reed Bunting.
My Flickr photos...
Lat/Lng: 50.7618, 0.1493
OS grid ref: TV516980
Cuckmere Haven, Countryside Park.
200m inland on bush on island, brackish water.
Seen previously on same bush.