if roof tiles are described as porous and water trapped below does this have to be a new roof replacement ,or is there an alternative course of action which would be less costly.

Although planning permission for re-roofing is not typically required for straightforward residential projects, building regulations approval is usually necessary when re-roofing a dwelling. Other permissions may also be needed depending on factors such as project scope and location. Also Once tiles become porous they have reached their expiry date as they become flaky and worn there are plenty of long-lasting and durable handmade towels which are very cost-effective hope this helps

if you have an underfelt this should stop water entering the house, but if there is water trapped below the tiles this will be rotting the lathes and the underfelt you do not require planning permision for a new roof. you do require building regs usually done on a builders notice when you replace more than 25% of your roof covering you are obliged to upgrade your loft insulation to current gov standards 270mm at the same time.

batons get wet and rot felt is only an insulation

When tiles are porous they absorve moisture & in freezing cconditions this moisture expands inside the tile, you might have flakes of tile coming of the roof, the only alternative is roof painting after a period of dry weather to seal the surface of tiles. an expensive cheap fix, much better to re-tile the roof for a little more cost & long term benefits