Having spent some time with the man from Dell (monty?) it appears we have resolved the issue with the autoloader. It now backs up across multiple tapes - hooray!
It appears that the Removeable Storage Media service was the problem; once it was stopped and the Dell drivers updated, the system was able to correctly identify the autoloader device; previously, it just showed it as an Ultrium 3 tape drive.
I carried out a couple of tests over the weekend and they both worked really well - I've now loaded the device up completely, erased all existing data on the tapes and am going to start setting up a more efficient tape backup regime. I did one test and it allows me to pull out individual files from the tape - I need to work out how long we are going to keep the backups and how often we will run them. We have a number of options, but I'm not certain which is going to be the most efficient method. I'll maybe post some more on this later.