Ledson signed a three year deal, so is out of contract in May 2019 -
https://www.oufc.co.uk/news/2016/august/oxford-united-sign-everton-midfielder/
It came as a surprise to me to find he only played 30 times last season - I forgot he missed the first month of the season.
Preston would be a brilliant move for him - closer to home, a chance to get regular football in the Championship, decent size club with a project going on. A much better move than going to a "bigger" Championship club and benchwarming 50% of the time.
So if there is a bid from them (It appears there has been) - then I think it will turn his head and unfortunately much like Lunny and Marvin, there will be little we can do to keep hold of him.
Best case scenario for us is that we can strike some deal where he doesn't move until the summer, whether that be by agreeing a half-season loan or by delaying any deal to end end of the season.
If they really want him now, they should pay more, simple as.