Hello Henry,
yes its a juvenile Stonechat for me too.
The extensive yellow bill gape, the clouded patterned feathers on the breast and the moddled ear-coverts are juvenile feathers/traits, while some new deep orange feathers on the belly are growing first winter/technically adult feathers.
Most similar 1cy Common Redstart has a orangey-red tail (sic), a different bill structure and slightly paler orange belly, not that deep brownish orange hue of your bird.