depending on how bad the felt is, i.e. if it is that trades people on the roof over the years have put their feet through the felt, small patches can be inserted in a v shape to the width of the trusses and stapled to the roof trusses internally over the lower strip and under the upper strip.
If it is a case that the felt has failed completely, the only correct way is to replace it, and I am afraid a roof refurbishment is often required after 40 years or more, refelted with a breathable roof membrain, not felt, is the correct way approved by building control.
Other methods of "tanking" from inside only costs more to replace the roof in the future when it finally reaches its lifespan.

In my opinion the best way for a quick repare would be to lift the tiles around the problem area, cut the batons back out the way and replace the damaged felt with a breathable felt. Then fix back the batons and relay the tiles.

Hi Graham_779, I'm afraid the blunt answer is no, if it was one damaged area then the relevant area can be repaired, but it sounds like it has perished all over and that is definately a case for removing and replacing the whole lot.

no.
no its not a cheap job it will require scaffolding and also requires building regulations done on a builders notice, you also need to upgrade your insulation in the loft at the same time to current goverment regs 270mm.
the size of the roof if their is any valleys or lead gullies that need replaced, type of roof tiles and if you are replacing the tiles with new or putting the old tiles back on will determine the final price.

good luck alex

You could always have your roof cleaned off then painted with plastic coated paint it cheaper option then refelt and battoning thanks