RC - Having now eaten a rather large humble pie with a Trilby side dish, you were absolutely correct in the first place ref my collision code being at fault.
Lost in the depths of time, I had a collision 'system' that worked for a world created directly in DBPro (ie seperate terrain & independent objects). Replacing these items using the .dbo file created in a map editor [as a trial] unsurprisingly required mods to the original collision code - but I failed to see it.
Thank you (& Neuro Fuzzy) for your assistance.
Habits can get you into trouble sometimes (unless you are a nun....)