An elderly relative has a lockable door chain for which the keys have
been lost.

http://i49.tinypic.com/350ju4h.png
http://i50.tinypic.com/9gdl6e.jpg

I thought it would be easier and cheaper to replace the whole thing
than get replacement keys, as a new chain with two keys was only £6.

Unfortunately, when I went to remove the old one, I discovered the
latch on the end of the chain prevents access to the lower securing
screw.

Can anyone suggest a removal solution that doesn't involve drilling
out the barrel, jemmying off the old hardware or calling out a
locksmith?

Thanks in advance.