you can put less insulation between the rafters and more under the rafters if thats possible in your case.saves the depth of under battening needed.
you will need the 50mm air gap no matter what.
you dont need to change your existing felt if there is adequate ventilation.adding soffitt...
beware of plastic cladding,i went to look at a job last week that had timber cladding changed to plastic cladding and now they have horrendous condensation problems inside.admittedly its a single brick construction.
why mend something that aint broken.i would felt your roof only when it leaks and if you dont touch the tiles then it shouldnt leak,assuming its not leaking at the moment.
If you do felt it,make sure you use breathable felt.
no membrane except on top of the rubble etc.then concrete on top 100mm wil do and then make sure you have enough topo soil on top if its in a garden area.you can use any bends you want ona rainwater pipe run.
i took my posts,turned them upside down,put them into the metal post holes,refixed the panels and cut the top off the top of the post.the post has to be quite a bit higher than the panel in the first place to be able to do this.
not seen the ones over here but i did fit a couple of external door/frame combo's or whatever you wanna call them in the states a few years back and found it pretty easy considering they were the first id seen.
we seem to be more american than english these days.
cant see much from the pics but i would make sur ei knew what was needed before committing to buy,its obvious the roof has failed at the point where the tiles are not sitting right but what the damage is inside the roof i dont know.
the gap between the chimneys could be a bodged attempt at...
my measurements were for standard size doors of today.didnt realise you had anything differnet.
measure your doors and add on 3-5mm for the head plus 2 thicknesses of the lining obviously.you can cut the height to suit your doors on site if you get them slightly over sized.
I.E.if you door...