Please can you tell me what makes it a cattle egret?
Yes that is quite possible. So do little egrets have a stage without the yellow feet? I am no expert on these birds, but is a little egret more graceful in its appearance and a longer beak?
For me this is a Little Egret, rather than Cattle.
Not only is the bill blackish, it also looks too long and slender for Cattle. Also it has white plumes starting to develop on the breast and back which Cattle Egret should never show (when a Cattle Egret does have these in the breeding season, they are orange).
The apparent lack of yellow feet could be the result of mud hiding the true colour.
Habitat is a useful clue but isn't exclusive - I've seen Little Egrets in dry fields a number of times, sometimes in flocks with Cattle Egrets.
Apologies for linking to photos of captive birds, but I hope it's OK for this purpose - some comparison shots of Little and Cattle Egrets together (in the aviary at Golders Hill Park in North London):
https://www.flickr.com/photos/126840895@N05/20262746143/in/album-72157655424945194/
https://www.flickr.com/photos/126840895@N05/20857653676/in/album-72157655424945194/
https://www.flickr.com/photos/126840895@N05/20697141269/in/album-72157655424945194/
Your bird is a Cattle for me on structure (and location/behaviour), but I am finding it odd that it has such a non-yellow bill... though Opus says "Juvenile birds have a greyish-black wash to bill and legs", so maybe this is a first-winter bird that's a bit delayed in bare part colour?
Actually, the odd thing is that the heavily "jowled" look of a cattle egret is showing here, with the feathering running well along the underside of the bill (see green outline added below); as commented already above:The very heavily "jowled" look of a cattle egret is lacking here. This is caused, in cattle egret, by deeper feathering running farther along the underside of the bill than in little egret.
Your bird is a Cattle for me on structure ...