Thank you. It was a quirk of blueprint scripting that it ran twice when attaching from a “float + float” node. I set a local variable to the result and used that in both cases and it worked. 
In my game, I can also lower the tiles using the abilities. So that the tile can go below 0 in the Z position. I have set the height map to be -500 to 500 on the Z-axis and the tiles are clamped within -150 to 150. Is there any reason would stop pathfinding from working for tiles below 0 in the Z-axis?
Just in case it is problem with the way I am handling updating edges, here is my UpdateTileEdge function: UpdateTileEdge Function posted by Drewbie | blueprintUE | PasteBin For Unreal Engine
The second problem, I am doing an AOE raising of a tile. If I do this close to the edge of the gird it raises tiles on the other side of the grid. I am currently using the GetTilesInRange function to find the indices of the tiles that need to be raised. Any tips on how to fix this?
After these are fixed my next venture is to have a unit which does not count toward the victory condition. This unit will be off the grid, will have a turn, not be attackable by other units and only usable by players. How would you suggest I remove the unit from the victory/lose condition?
Kind Regards,
Drewbie