Xara support got back to me with a direct link and it works fine - once I had corrected the link given to:-
a. Work - there was a gap in the URL that threw errors as you might imagine - and
b. Direct me to the correct product; the original was for Designer Pro 7 which didn't help a lot! Luckily I was able to second guess from the structure of the URL given what the correct one was.

I think the upshot is that for certain systems the Xara Download Manager is not up to the task.