Roof experts - how much of a problem does this look to you?

[CryingInFrontOfStrangers]

We have serious damp problems upstairs, and a crack in the ceiling which I'm sure is getting worse. I've taken some pictures as I'd like some initial opinions on what the problem may be before for my next conversation with the landlord.

Pics of the roof

Pics of the ceiling crack.

The white stuff on the rafters looks like possible rot. Are they solid all over, or spongy feeling?

They do feel damp but not particularly spongy.

Is that crack in the ceiling where there is a gable end wall? If so, what does the wall look like from outside?

Are these in two separate places? The crack is nothing to worry about. Small cracks in plaster are normal and not structural so don't worry about those.
The rafters don't look great though and could do with checking.
(Work in public housing so not an expert but familiar with housing inspections)

Your roof needs re-felting. You have no waterproof layer left under your tiles so you will need your roof doing asap. Sorry.
Tiles last longer than the old felt they used to use and starts rotting after about 30 years. A product called Spamtec has a half shelf life of 75 years and will last longer than the tiles.
If your tiles are still good, you might just get away with just replacing the old felt.
Not cheap and good luck.

Not sure If that spamtec thing is a joke?
Anyway roofs of that age were built with plain tiles and no felt or membrane. That's fine, but over time the torching (cement on the back) can fall off and tiles can move or crack and let water in.
I think your roof at least needs a survey and tiles replacing, and may indeed need replacing with a new buildup.
Regarding the movement, houses do move through the seasons, however progressive movement would indicate a problem with the roof or something supporting it.
Either way it would be worth getting either a surveyor or roofer to have a look - or your landlord should if you're renting. Good luck.

Sorry, spantech.

Why are the tiles white? Have they been sprayed with insulation foam at some stage and its all cracking now? I believe that stuff is a nightmare to get off and as said above you need to re roof with proper roofing felt and probably new tiles.

The white stuff on the back of the tiles looks like cement and is crumbling away (the loft and everything in it is covered in it) so it sounds like what Johnd2 said it is.

Thanks very much for the advice, I feel in a better position to talk to the landlord about it now.

I agree with Johnd2. It might be worth checking the surrounding wood for signs of rot. A bit of damp is fine, it'll dry after the roof is sorted, but rotten wood ideally should be replaced.
I think the cracks on the ceiling will be due to the infiltrating water, rather than anything more sinister.