It may be what the roofer says, but I think it is more likely that condensation is forming in that area, simply because the edge is the coldest part of the ceiling / wall. It is the coldest simply because the insulation is less good there and it is closer to the roof tiles.
I'd be inclined to have Celotex/Kingspan put between the rafters in those area as first, making sure the depth of the material still allowed air from the vents you should have in the eaves to pass over the top and ventilator the rest of the roof.
Taking the tiles off a 1960s roof to change the non-porous felt for a modern breathable underlay like Tyvek sounds an overkill to me.