I am writing this just to let others know in case they come across the same problem.
I have spoken to both Celotex and Redland on the correct set up for the roof covering.
If the rafters are partially filled (so you have an air gap) then you use a breathable felt, but there is no need to counter batten. However if it is a complete fill then you need to create a breather gap, so you will have to counter batten the rafters before battening them out for the tiles.
If you are battening directly onto the rafters then you need to create sagging of the felt between each rafter for drainage. There is no need to do this if you are counter battening.
As this is a vaulted ceiling roof I need to create ventilation at the ridges and Redland told me that the whole length needs to be vented using a dry vent system.
The roofer I've been using always pulls the felt as tight as possible and then battens straight over the top. He also said that 2 or 3 ridge vents would be plenty. I don't know if Redland are going overboard, but I'd prefer to do that, than not do enough and it cause problems.
It doesn't give me much confidence in using him again.
Cheers.