That looks like trad roofing felt. You’ll want some more of it and some bitumen adhesive. Stick a patch over.
That’s crap news on the rafters, tho it looks like they’re not rotted back to where they’re notched onto that plate so hopefully you can ‘just’ cut off the rotten bits and add new.
@Howard @hugo7 @hoefla
Thanks for the replies.
I've been doing some loft boarding, and there are a few tears in the ?felt where it flaps down:
Not having much luck at the moment, as one of the fascia boards broke off in the high winds last week, only to reveal a rotten rafter tail: