I have been prototyping a simple navigation app using QGIS. If I generate tiles from OpenStreetMap using Generate XYZ Tiles, I create a basic app using Leaflet JS. I can then use the Geolocation API to monitor my movements. I am using EPSG:3857. However, I need to use a base map generated by a third party using a CAD tool, in order to have more detailed features than is available in OSM. However, their CAD tool only supports Eastings and Northings (in metres). Once I have imported their layer group, and set the CRS to EPSG:3857 I still have the position information in Eastings and Northings.
I have tried to change this but cannot find any way, but it surely ought to be possible as I can type an individual location into, for example, Grid Reference Finder, and I get back the correct lat and long?
Addendum 10th Feb:
I think I am getting to the root of the problem. I am doing something stupid but not sure how to fix it. Let me explain what I am doing and then I will be able to rephrase my question.
If I start off with an Open Street Map base, and then try to add my DXF layers, nothing appears on top of the OSM layer, although the DXF appears to have loaded without a problem and its three layers appear in the Layers window. So, what I did was to open a new project and simply add the DXF. This is what I get: Notice the coordinates at the base. These are in metres. However, the origin was presumably set in the original CAD file so that these are correct eastings and northings. This is why I thought that the CAD file had been located correctly. But I don't believe QGIS has done anything of the sort, which is why my DXF would not register on the OSM map when I tried this. If I then add the XY fields (or just right click on the map window and copy coordinate) I get something like this: 4.548068,1.431102. It is way off the true location and I believe this is because the lat long location of the imported DXF is quite arbitrary.
So, it seems my question should be along the following lines. How can I create a custom CRS given that I know the mapping of the metre coordinates onto Lat Long? For example, I have been given that the four corners of the box around my map are: Top Left Corner X = 505900.0 Y = 160200.0 Top Right Corner X = 507400.0 Y = 160200.0 Bottom Left Corner X = 505900.0 Y = 157500.0 Bottom Right Corner X = 507400.0 Y = 157500.0 These are eastings and northings so if I convert them using Grid Reference Finder I do indeed get the correct lat and longs.