Thanks guys, you've all been so helpful. As it happens RileyP, I do in fact have a tiled roof in the existing house, and I intend to have the extension in colourbond and to replace the entire old house roof with colourbond at the same time. I have a number of reasons for this;
1) I LOVE the look of colourbond
2) 45 year old house, roof needs re-pointing, gutters falling apart, and fascia boards all rotten so large-scale changes required regardless
3) It would be impossible to match the old tiles anyway
I also think that with changing roof cladding, it should make it easier to account for some slight variations that will no doubt appear on one side of the house, where I am following the exisiting roof line. The process of re-battening and so on should allow for some fine adjustments to compensate.
I shall be very meticulous in measuring things for the truss manufacturer, in particular the profile of the rafters to ensure the roof line doesn't vary.
How long does it typically take for the walls/trusses to be built?