Buggered if I know what i did, but when I went to resume the download, I got a 'Complete, 1652.6 Used' message and a 'Delete' option straight away, and it appears to be working without problems.
Just for grins, I deleted the Topo map and re-downloaded it and it behaved as it should, so NFI what happened the first time.
Now onto the compliment...to whomever thought it would be a good idea to include the 250K Topo map, it is. Well done.