I didn’t think this was going to be so troublesome! but wanted to note down the steps to resolve.
The ‘Build Virtual Raster‘ process in QGIS will not bring together multiple rasters if they span across 2 CRS projections
Following my initial post of the issue on GIS stackexchange:
https://gis.stackexchange.com/questions/342830/cant-join-multiple-dems-using-qgis-build-virtual-raster-if-they-span-2-crss
A users tip re unifying the projections for loaded data sent me in the right direction.