How to Map f2 to Ctrl u in Excel (to make cell editing easier)

Just upgraded to High Sierra on my MacBook air 2011.
Unfortunately my Excel keyboard shortcuts have stopped working – I was using Karabiner to map F2 (usual windows edit key) to use ctrl u, which makes it easier to use excel.
I had to also move to Karabiner-elements, as the version I was using isn't compatible with High Sierra.
I have attempted to write a .json file to setup a complex_modification, but it doesn't seem to work – the event viewer just shows the f2 key
Im using MacOS 10.13.6
Excel for Mac 2011 version 14.7.7
Karabiner Elements 12.6.9

The code I wrote is:

{
    "title": "F2 change to Ctrl U",
    "rules": [        
        {
            "description": "F2 change to Ctrl U",
            "manipulators": [
                {
                    "type": "basic",
                     "from": {
                        "key_code": "2",
                        "modifiers": {
                            "mandatory": [
                "fn"
                ],
                "optional": [
                         "any"
                        ]
                        }
                    },
                     "to": [
                        {
                          "key_code": "u",
                          "modifiers": [
                            "left_control"
                          ]
                        } 
                    ]
                }
            ]
        }
    ]
}

What am I missing?
Thanks,