BandLab have responded and I have discovered something I hadn't realised. Several years ago, I bought "Music Creator 2" by Cakewalk and still have the original installation disk. I installed it but was still able to run my Atari setup so never used the MC2 software although it's still on my computer. Following BanLabs' instructions, I have their version installing - it seems I had to tick all the add-ons before clicking <Install>.
I'm now not sure if I'll have two versions of Cakewalk available and whether they will be identical or not. Once the installation is complete, I'll give it a run-through and see if it does the job.
Ideally, I would like to access the old "song" files the I created with Breakthru' but I suspect they'll be in an unusable format and I'll have to pursue the Atari emulator (STEEM) before I can use them. It looks like that will involve creating a disk image file to use within STEEM and that fills me with dread - it seems to require an internal floppy drive and my external (USB) drive will not work. That means buying and internal drive and, horror of horrors, delving into the guts of my computer to install it - I could end up with a busted PC!
As far as Cakewalk is concerned, would you be able/willing to help if I need assistance?