The last few days at work I've been renovating an old roof. In the end I think it would have been easier to have taken it completely off and start again but never mind. At least we didn't have to alter the purlins by doing it this way (although some of the rafters had to be packed to catch the purlins).
On one side of the roof I replaced every rafter due to rot and woodworm whereas on the second side I just extended each one down so it will cover the future external insulation as these were all sound. Where the rafters were replaced everyone had to be cut to a different length due to the building being out of square, this made it a bit of a slower job, but its only a small roof!
I decided that although the specifications said to insulate between the rafters and on top I made it a "warm roof" and insulated only on top, this keeps the air flow between the rafters but used a more expensive insulation to meet regulations - the building inspector was happy with this solution.
The roof insulated and counter battened, ready now for felt, batten and tiling.