I don't think Utd would be in the free April midweek as I have a feeling if its postponed Sky are going to pick the Villa match for the Monday night slot, that way they can engineer the Utd match for the last two weeks of the season. The derby is likely to be the Saturday following as its either going to be a BT or Sky first pick with the other probably picking Arsenal v Utd, so that would leave the April midweek impossible. Weds 11th May is my best guess for Utd, and I've booked that day off work in case, though it could just as easily be the 18th May. Either way we won't know until after the FA Cup 5th round is played as they said this:
Due to the number of outstanding Premier League matches still to be rescheduled as well as other potential postponements as a result of the FA Cup Quarter-Final draw, only the opening two match rounds in April will be announced early next week.
The rest of the month's broadcast selections will be confirmed following the completion of the FA Cup Fifth Round.