Rich, dug out the Tyvek and cut a 8ft length of it, its 1m wide as well. Total weight of sheet came in at 290g, which lets be honest isnt mega light, the stuff ive got is the roofing version, which i think will be a heavier cloth than the building paper version. I will check it out next time im in the builders merchants and see the difference for you.
Its yours if you want it (oh and its green as well), if not i will try to see if i can get hold of the lighter weight tyvek....
I'll call into Wickes tomorrow and check out that glazing sheet, if it's not up to much then I'll give you a shout