Thanks d deweywright, thanks tsantee,
How and when my Toast 6.0.3 Titanium went wrong is still not clear, but things are happening.
I have managed to buy a boxed copy of Toast 6 at auction and as soon as it arrives I will create a Mac Archive of the contents and key and send it to Corel so that they can make it available again. When I have done that I will confirm my action and quote the full version number.
Having also purchased the necessary equipment, I will set about examining older hard drives removed at various times as software increasingly became bloatware. I expect to find working installations on several of these. The drives themselves range from 10 Gb to 800 Gb, figures I mention solely to mark the ever-upward appetite of software developers.
If I ever used an earlier version of Toast I shall try to rescue it, but that is less likely. We would be getting back to a time when Syquest disks were the normal means of transferring information between designers and filmsetters in the UK.
I did have one unexpected find that raises my hope of unearthing all my software master disks eventually. When I opened one of the big heavy boxes packed by the removals people and marked 'Kitchen'. I found what I was looking for ... and down at the bottom and the source of much of the weight was my venerable Mac Plus and its 20 Mb Hard Drive ... and that size is NOT a misprint!
I hope this is good news for others.