Sorry, being dense!
I’ve downloaded a couple of other programs and tested these. One of them cannot be made to do this at all, but it always unloads the source after scanning, which I can tell because loading the source causes its splash screen to display. The other program can be made to do this as it only unloads the source when told to do so.
What I don’t understand is that if I send the command enable source to the scanner using the WIA-HP Scanjet 4800 driver, a scan is produced and the source’s status is set to disabled. If I then set the source to enabled again, a second scan is produced. Using the same procedure with the HP 4800 Series Twain driver, the first scan is produced as before, but setting the scanner to enabled a secod time, does not cause the scanning process to take place with the previous scan simply being downloaded again. With this driver, the scanner must be unloaded and re-loaded between scans. I assum this is a solution, but I’d like to know why this happens. The HP UI does not unload the scanner between scans, as the splash screen does not display between scans.