* Scaffolding - £7k-ish.
* Roof - £20-25k for slate.
* Assumption - No structural work required.
Obviously is depends on height of building and size. I've had a guess based on the prices we got quoted for our house.
Factor should take care of it, that's what you pay them for. Their should be a maintenance fund that they use, but if that is not enough then there may be additions on your normal bill.