OK Solution from Pierdek was to adjust the settings for APEXMaxDestructibleDynamicChunkIslandCount / APEXMaxDestructibleDynamicChunkCount by placing the following in DefaultEngine.ini:
[SystemSettings]
p.APEXMaxDestructibleDynamicChunkIslandCount=20000
p.APEXMaxDestructibleDynamicChunkCount=20000
p.bAPEXSortDynamicChunksByBenefit=1