After even more trial and error I was able to narrow it down to just this line, the removal of which allows the file to display correctly: xmlns="http://www.w3.org/2000/svg"
it can also be fixed by changing it to xmlns:svg="http://www.w3.org/2000/svg"
Which looks to me more likely to be strictly valid and correct and I hope is the actual fix. Could someone please tell me if it is correct so
Hmmm.. not quite. I think it's a bug in their viewer.
That first statement says "if no tag names have a ':' in the middle, default them as belonging to the namespace {http://www.w3.org/2000/svg%7D"
The second says "if tags names have a ':' in the middle, and the first part is 'svg', then and only then take those elements' as belonging to the namespace {http://www.w3.org/2000/svg%7D"
So... given that none of the tags in there start with "<svg:", that second statement pretty much is equivalent to deleting the xmlns attribute. The way present in the files just sets the default namespace. It's quite bad that it presents a problem, since among other things, that namespace is what the 1.1 SVG spec uses (like in section 23.1).
For info on default namespace, see 5.2 Namespace Defaulting of "Namespaces in XML" http://www.w3.org/TR/REC-xml-names/