I've just had a survey by a cavity wall insulation company, they're
one of two approved companies under our local council's insulation
grant scheme. I'm confused over a couple of things they've said hence
this post.

I have a detached garage that is approx 4-5 feet away from one gable
end, they say that with this space for a ladder they can't safely
reach the roof apex and I'll have to pay extra for a scaffold
platform. I asked how high they could go on a ladder with the space
available and they said up to the gutter line. Not being an expert in
this field I'm wondering why do they need to insulate above the gutter
line anyway? Is there any reason why I need to have the wall cavity in
the loft insulated? If I don't is there a risk of damp?

My next query is about our lounge which is 26 feet by 14 feet. It has
a gas fire with a normal flue and the room is double glazed with a bay
window at one end and patio doors at the other, the patio doors are
fitted with a permanently open trickle vent. The surveyor has told me
I'll need a 150mm square wall vent installed at extra cost. The reason
given being that CO2 produced by the gas fire won't be able to
permeate through the wall once the cavity is insulated. Is this
correct (and why) or just an unnecessary 'extra'.

TIA