Slightly heavier option than suggested here.160g ish
I used one of these for a Ryanair flight this year and used it for a groundsheet in my porch.
https://www.ebay.co.uk/itm/192663741106

Alternatively and cheap I know people swear by these laundry bags

https://www.ebay.co.uk/i/112526960659?chn=ps&var=412913767026

Cheers guys i'll take a look at all three links

appreciate the help :+1:

I've flown with my backpack from the USA to the Uk and Europe more times than I can remember and have, in the last 10 years, used one of those zippered IKEA bags used to store duvets and the like. They're £3 and I have sent them on to Montrose for the return if necessary. I even shove my poles and boots in these bags along with the pack. It's not like I can go anywhere if I'm wearing the boots and the oack doesn't arrive anyway. Yes, they're see-through and I just bungee around it or tie a string. I carry a plastic carrier bag with camera, compass, important/small expensive items. Never had a problem.

This bag, any bag, will get scuffed up during baggage handling. This is not the time to go lightweight. When backpacks were made with thicker, more robust materials you could get away with securing the waist belt backwards around the base of the pack and let them take it that way.

Good luck!