I snapped the rubber strap that holds the coolant bottle. I cut up an inner tube and made a neat join with rivets, maybe you could do the same if you don't find one
That's a shame - it's always a pain when sites advertise parts they don't actually have!
I found this on a French site - again, no idea whether they still have it, but it's worth a try. There's a secondhand one on the same site. Best to check the part number too.
I'm pretty sure the early Mark 2 fives (and the derived vans) used the same strap - might be worth a look in your local scrapyard. Or maybe someone going to Thenay could keep an eye out for one there?