Yes, it is a pitched roof, and I am talking about the felt between the
rafters and roof tiles (concrete, I assume).

Most Victorian houses don't even have felt. Certainly, if water is getting
past the felt, then you need to fix the roof, as there should be no water on
the felt in the first place. The felt just provides a second line of defence
(especially against powder snow).

Replacing in-situ felt isn't easy. You basically have to take the entire
roof covering off, complete with battens. At least that is how they did it
on my loft conversion before relaying breathable membrane and reclaimed
slates.

Christian.