I've called the vet and had a good chat with one of the nurses, who read Lorna's file and considered her progress. She's going to enter a transition phase with mash available for grazing and four weigh-ins a day. If a weighing shows shes lost more than 30g, I'll feed her. Otherwise, she can continue on with mashes and dry foods.
The nurse also said it's normal for weight to vary by ~20g for a chicken Lorna's size, as it's possible she's about to poop or has just had a drink, etc. Overall, Lorna's progress is good.