I am continuing to investigate with ED if there is any way round this. One idea I had was to maybe have a different map file loaded depending on which date was chosen. I,e, pre 12th May 1985 would load one map and post would load another, whoever, currently this feature isn't available with DCS at the moment. I have requested the feature be added.
This would make the actual download of the map a lot larger but that's the price you pay for having essentially 2 terrains.
Fingers crossed.
Thanks Specter