Holm Oak Leaf Dropping

Hi all,

we have a very old Holm (evergreen) Oak tree in our garden, that started shedding its leaves about a month ago. The leaves that have fallen are brown and patchy, many with signs of something entering or escaping the underside of the leaf, inside the leaf is dark brown. The tree has also produced new leaf growth which seems to be ok and pollen catkins which have now fallen. However all the dark old leaves left on the tree are patchy brown. Is this normal? Could a leaf mining insect have caused this? Leaves and catkins are continuing to drop on a large scale daily.