It does sound like Sweet Itch ( hiya Bionic
).
Nip into Boots and pickup some Sudocreme (prob in the baby section, it's usually used on baby's bums!).
Apply a liberal amount to the top of tail and underneath top of tail. Massage it right down into the hair at the top of the tail.
I also rub a bit either side of the top of the tail, onto the skin (i.e. either side of bum).
If your equine is anything like mine they will enjoy the massage.
It will help alleviate the itch, help sooth any raw spots, deter the flies that cause it and help stop the tail breaking away (from excessive rubbing).
NB: Rub a bit on the other raw spots too. If it gets worse, or bleeds, prob best to get the vet to check it.