Your best bet would be to ring Spillers, since you're using their feed, and ask their advice. The photo you've put up isn't a great one for assessing condition, but she does look to be fairly well covered from what I can see. Maybe get some side on shots with her stood properly that you can send to them, and a rough idea of her weight on a weigh tape.
I'd try to give her hay every day, even if it means you put a bit less out each day.