My replacement inner tubes arrived today. The courier tried to hide them in the cupboard containing the pump for the well, but I found them eventually. It barely took ten minutes to get one of the new tubes back into place. Then I had to pump it up without the valve in just to get everything to settle into position, after which I was able to replace the valve and get the tyre up to an adequate pressure.
Not sure that tyre’s in great shape though. I think it will need replacing at some point in the relatively near future. It will do for now though. It’s not as though the wheelbarrows get used for much that’s really heavy.