I'm very glad to see these opinions because I took a fair bit out from the upper portion of the roof to get better access when fitting rockwool loft insulation on top of that granular stuff you can see in the pic. Actually the second layer made it slightly easier to fit because I could go right up to it without fear of blocking off ventilation. When I peer down between the two layers there is still an air gap. Thanks all.
My loft in a 1920s semi-detached house has the conventional layer of felt over the rafters and under the tiles but in some areas also has a second layer of felt loosly tacked to the underside of the rafters. Much of it is coming down as can be seen in the attached photo. What is the function of this second layer of felt ?