Decided to go with DrRingDing in the end 'cos the filo was looking a little bit damp and had lost its crispiness, but full on oven heat would have burned it. Plus I don't have the patience to warm things through in the oven, takes too long.
Anyway, 3 minutes in the microwave and then 20 mins covered in the oven at gas mark 4 and 5 mins uncovered turned out to be perfect.
Obviously not as good as freshly baked filo, but not bad considering.