Apologies for the lowbrow nature of my problem.
I enjoyed Doom, Heretic, Quake etc. plus all the extensions first time round and was delighted to discover the Doomengine method of playing them on a new computer.
Managed to get Heretic loaded and running all except the very last boss level.
Have since tried to load up Doom2 and failed. When Snowberry launches it only goes through part of the procedure then finishes but won't launch.
It won't even ask for the WAD file.
Any suggestions greatly appreciated.