For ages now, Ubuntu has been failing to apt-get update properly. I have finally tracked down the problem, and it is the useless “transparent” proxy that my ISP (NTL) use. It has totally broken If-Range support. Basically it doesn’t return any HTTP response headers, and so apt gets confused.
I have raised an ubuntu bug in the hope that an option can be added to avoid using If-Range. And I have sent an email to NTL to let them know of the problem. I’m not at all hopefull that NTL will get it fixed, but perhaps the Ubuntu peeps will take pity on me 