In reply to marsbar:
Thanks for the replies, yourself and others.
I got some 1 cm wide elastic tape from Abakhan in Chester today - 50 p for 2 metres! It will do the job, though it doesn't look very durable and may wear out fairly quickly. Now that I know where to get more when I need it this isn't an issue.
The equivalent elastic tape on my older (5 years?) Wild Country harness was wider, 18 mm, and lasted a lot longer.
Perhaps in future I should make more effort to avoid excess wear by not descending steep, wet grass on my bum, but on a recent trip to Lundy this seemed to be by far the safest method...