May be of use
http://www.projects.toolvolt.co.uk/listing.php?id=001
But make sure the vents are clear in the soffit and that thay do go right throu into loft and not there just for decoration.
If you have small round vents in your soffit that are right throu, then increase the amount, manufactures recommend every 200mm all the way round. Which equates to a 20mm slot vent all the way round.
If you have a high pitch roof, fit 2 tiles two thirds the way up on all sides of the roof.
www.toolvolt.co.uk, February 2010