Weird. On about the fifth attempt it now seems to be working OK (having removed disc, cleaned etc. each time).
Possibly a message for others, keep persevering!
Thank you for the reply. Slowly but surely it seems to be sorting itself out and installing. Just took a number of further pauses/removing/reinserting the disc to get it to work.
Apologies I did try Googling but should have also tried Reddit. If I’d known it was a common theme I would not have posted here but thanks for the link!