Just one other thing as I quickly ran past the previous posts,
Hardrow Slates are normally hung with alloy pegs, but if they are nailed then they can easily be lifted by just lifting up the slate by the bottom of it and it will remove the nail without damaging the slate.
yep Hardrow slates, they will go down to 17.5 degrees, see here: http://www.forticrete.co.uk/products/141/hardrow-solo-roof-slates.html
But only with the correct headlap!
so maybe you can make use of them and just introduce an extra course or two?
You can use them but they are for general use and are normally the strength of brickwork mortar which is usually a 4/5 to 1 mix and as stated earlier roofing mixes should be a 3 to 1 mix.
Why would you want to do that?
What is wrong with putting soffit air vents in?
personally I would have used 75mm Celotex boards in-between your rafters as a 25 mm air flow is all that is needed.
Dont forget the rule: Condesation is solved by Insulation and Ventilation.
Also dont forget...
The last winter freeze caused a lot of ice build up in many gutter systems and nearly all of them had huge icicles hanging from them.
I had many call outs because of this and most of them were because the plastic guttering could not take the weight and had come down or was hanging down...
Firstly you do not re-point anything in roofing, it has to be taken off, cleaned down and re-bedded.
Secondly do you have insulation over the loft hatch? It could well be condersation.
On closer inspection it looks to me like the valleys are old zinc (i could be wrong) and leaking (for which I am sure).
The biggie is that you have small slates on your roof and to repair this is a nightmare! it can be done if you accept that a lot of slates are going to be stuck together with...