I'm trying to create a map with the co-ordinates for the location but if i select map units it creates black boxes in the corners of the map instead of a line of co-ordinates. If I used centimeters it puts the co-ordinates in the correct places but they are the same for latitude and longitude which is incorrect. I have ensured that the CRS is set correctly for the grid and the map and I'm struggling to work out how to rectify this. Below are screenshots to show the output I am getting.
When you chose map units for your grid they are in metres (like the OSGB Grid you are using) so you need to add a reasonably large spacing - I like 100,000m (or 100km) like this:
Or you can use Lat/lon by changing the crs of the grid to EPSG:4326 and using a 1 degree separation like this (notice how the lines are no longer straight):
You hardly ever want to use cm as those are just measured along the page and have no relation to the map whatsoever.