Thank you again for your suggestions.
Amazingly a colleague of mine has sourced PGPfreeware 6.5.8 which supports CAST, IDEA and 3DES. It also provides command line functionality.
A local university (we give them free use of our scientific instruments) had a copy in their computing faculty.
We’re going to give it a try but if that fails then I’ll drop you a PM. We have original installation media for Windows 3.11 and MS-DOS 6.22 and we probably could source Turbo C (if we don’t have it already somewhere).