You were right. one of the tiles had slipped down leaving a 2/ 3 inch gap. It appears its been like this for a good few years, leaving the felt to do all the draining away, the birds nests held a lot of water back over time and helped in the process of rotting the felt. What beats me is how this tile as managed to get over the batten with the weight of these tiles, as there is a good lip on them, ( unless it missed the batten when the house was built) But many thanks!
Yep lots saved, Young chap came from about 6 mile away, said he'd pop down, showed me from inside the loft first, took away some of the rotten felt, then got ladders up as it was right near gable. Only here for about 30 mins, asked for £25 but gave him £50. will look into felt job in summer. :0)