Thanks for the link, that did steer me in the right direction, but sadly no cigar.
I ended up calling Dell Suppport in the end and spoke to one of the UK Support team - David Sherlock (was very good!), and he identified that although we could see the individual tape drives in windows device manager, the Dell driver hadnt automatically added the tape changer device (I was unaware of how to do this, nor aware it needed to be present).
As soon as this was done, the device changed from being a "IBM LTO XXX" driver to showing "IBM 3575 TLC 30" and Veeam then could just see a single target in it's Backup Infrastructure window and BINGO, all the tapes.
Thanks Veeam / Thanks Dell / David.