I decided to start this topic because I forget stuff and I'm sure others will want to try to convert their maps to ue4 at some stage.
If you have different experiences or better methods, please feel free to post them here. Eventually I'll use this topic for a wiki page.
Stage 1 (Radiant prep)
Load your map into radiant and save it with a new filename. You're going to delete a lot of "stuff" thats not going to be needed and you probably don't want to spoil your actual map.
Delete all entities, models and clipping. Delete any brushes that are sky and of course any caulk hull that is 100% caulk.
You'll need to use radiant 1.5 for the next stage as the brush to obj plugin was removed in later versions (it's not included in 1.64 of radiant)
Select all the brushes in your map and run the export to obj plugin, don't forget to add the .obj extension as the plugin doesn't do this.
Import the obj into blender. Select everything (press A until everything is selected) then press Ctrl-J to join everything together.
Now we need to remove the rotation (X is set to 90 degrees) and to re-scale the entire map to 2.54
You'll also need to flip the normals at this stage, the option is on the right hand toolbox, uv mapping.
Now enter edit mode and start selecting faces which are smallish blocks of mesh which should be joined.
Split these from the main mesh (select the faces, press P and choose by selection). At this stage I normally assign them a material that describes the portion.
Eventually, you'll have something like this.
Once you've completed this stage it's time to export everything to FBX, I generally export only mesh, I choose smoothing by face and disable all other options.
You can now import this into UE4, I'll document this stage later.