Sweetitch is an allergic reaction of the skin usually in risponse to the culicoides minge. Like all allergies it takes time to develope and it is more likely that the midge is more prevalent in her new home surroundings than it was in the area she moved from. I think this is more likely than irritation from the hawthorn or reaction to tree bark or sap. I would suggest however that you give some attention to any marginal trace element and vitamin deficiency although you are certainly giving her a good mixed diet. (maybe possibly too 'rich'?).
As far as controling exposure to the midge which comes out at dawn and dusk you need to put her out at 10am and bring her in at 4pm. The stable should have fly screens and fly papers/insecticide strips or lights fitted. Complete fly proof sheets/rugs etc may help as does switch or some similar preparations provided the application is commenced whilst there is still a good covering of hair. I always used to expect the first case of sweetitch on the 15th May. It will vary from area to area and perhaps get earlier with mild winters and global warming. This means that control measures need to start by mid April.
Once the itching starts and the hair cover is lost then barrier creams such as benzyl benzoate will help but these should be combined with the control measures already detailed. Feeding large quantities of garlic, sponging down with cold tea and washing up liquid are also supposed to be of beenfit but I am not convinced. Gamma linoleic acid supplementation (borrage or oil of evening proimrose seems to help in allergic hair loss conditions in other animals so may be worth a try. If it works please tell us all. Good luck.

13/2/06