OK, I found my problem. The downloader aget is stuffed. I have since now gone back to wget and all is OK..

I had downloaded the iso using aget. I had cancelled it at the end, as it had gone into a download loop. The iso seems OK, in that I could mount it with the loop option, burn and boot from it, but it actually had a bit or two missing.

The moral is : do not trust aget for downloads. Anyway wget got the iso just as fast.