A lack of ventilation within roof spaces where impervious sarking felts have been used has always been a surveyors pet hate.
As a fifty something surveyor, I consider myself always open to learning and this just seemed to me so simple, – but so effective!
A little block of wood cut and then pierced with a nail, can be used to wedge open the felt overlap in the roof and let the air in. The nail simply stops it sliding down between the felt layers. One inserted between every rafter and at every felt overlap more than adequately vents the roof space. No need for difficult external roof access, no damage to tiles while expensive vents are inserted and so, so much cheaper! Ingenious!