My mate Dave came round last night and had a poke around in the loft. I'm very relieved to learn that there is no evidence of squirrel in the loft, and the birds' nests are between the roofing felt and the tiles. We've decided to leave them be until the birds fledge, and then Dave will fetch his ladders round, remove the nests and bung up the holes. There was also a small, extinct wasps' nest.
I really must get round to boarding my loft though. I had visions of Dave plummeting through into the bedroom. He's a bit braver than me!
So, big thanks to Dave and Jules for coming round to have a look and making me feel a whole lot better about it :o)