At 28 years old there will be nothing wrong with your timbers also yes that type of roof felt does go chrispy after a while I have the same at 50 years old, if I was to poke it sharply then I could put a hole into it --- so I dont poke it.
At the end of 2013 I lost 8 ridge tiles from wind damage and took it as a warning and had that whole hip run and top taken off and re set.
Now the whole roof was not 50 years old because 7 years ago we had an extension to sharp hall and me thinking it a good idea to take the opportunity to re set all the ridge tiles, and that was my mistake because the roofer was shit and the mixture was far too weak and ended up turning to something like chalk, I had been battling flaking -blowing cement up there for at least a year.
Your valley sounds the same.
I did a lot of research and asking roofers and this is what I found.
Ridge tiles need a very strong mix, something like 1-3 and not only that, the best mix is to also use sharp sand in the mix, so 2 sand 1 sharp sand 1 cement, or even 50/50 sharp/normal sand.
So one of the problems on a build is that sometimes the roofer just uses the mix that the brickies are using which will be 1-5 with no sharp sand which leads to yours and my problem.
I have since had the rest of sharp halls ridge tiles taken off and re bed and to every roofer I asked for a quote from, my question was "what mix do you like to use"
About 5 years ago we also had a single story extension to sharp hall which was suffering with the same soft flaking ridge tile mortor, so last year I did the focer myself - took off all the ridge tiles and 2 valley tiles and reset in 1-3 but all sharp sand. And took the time to spray the tiles with water so as to not suck all the moisture out of the cement too quick.
One of the reasons that quote can be more than you thought is that you will have to have scaffold which is adding something like 500-700 onto the cost.
After years of accepted methods, the build regs have changed to say that mortor only ridge tiles are not to be used - so they must also have a
macinacal fixing (screw or band system )
or be 100% dry fixing
Oh and another thing, roofers push up some of your tiles to expose the roofing laths which gives them some steps to stand on, then what happends is their big clumsy size 9s toes pokes holes through your brittle roofing felt