Sorted with Update Manager...
I have just tried to "apt-get dist-upgrade" from 8.04., hoping to end up with 10.04
However, the result is I am still running 8.04.
(Synaptic does not show any link to upgrade to 10.04)
I now only have 2 screen resolutions available, 800 x 600 & 640 x 480, I had the 1280 x 1024 resolution previously.
Some of the packages could not be downloaded, , "--fix missing" made no difference.
Rebooting from cold has not made any difference.
AMD Athlon64 2-core, 2Gb RAM, HP flat monitor