Thank you all for your input.
Yes, her tail is short HB, the wound is at the base of the tail (where it connects to the rump) so she can reach it - as a Soay she is small and bendy!
Definitely no other flystrike patches and no more maggots in the wound, have checked several times, and the fleece is off halfway up the rump. She is definitely biting the wound itself and not chewing elsewhere.
Will try and get some Stockholm tar in the morning...