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The condensation won't be down to the felt, it will be down to a lack of ventilation, perhaps someone has installed extra loft insulation and gone right to the eaves, new uPVC fascias/soffits can also cause issues as they seal better than old wooden ones.
The condensation could probably be sorted with a few vent tiles, lap vents or soffit vents (or a combination of all).
My brother's house that was built early 1900s has the original roof and no felt at all!! I think there are still quite a few houses around like thatm
Not in London but I wish you luck, the amount of roofers I've had over to quote for the same thing and then you never hear from them again is a joke. They all make a point of "measuring up" and saying "straight forward job" which makes it all the more confusing.
My theory is they'd all prefer to fix a tile for £200 x 5 a day. Makes sense, it's just a pain for us that need bigger jobs doing.