Well thanks for the replies folks
. She's still there and OK.
Putting her in with mine was a thought rather than a considered plan; That said, assuming she's from one of the near fields I wouldn't expect her to be high risk. Catch and quarantine is my preferred plan ATM but I'd probably need to organise a group to catch her - she's not going to starve where she is as long as she doesn't get stuck.
It's not a huge area of forestry and there are only a few adjacent properties (and no public roads, so the dumped stolen sheep scenario seems unlikely), so I'm still hoping she's one of the neighbours', and the message that there's a lost sheep will get through to them.
I do have a number for one of the forestry chaps though - somewhere - so I may give him a call.