I went and saw my VW mechanic cousin who I thought had the same key for his Y plate Leon.
Turns out while it looks the same they are actually ever so slightly different
Planning to call in to him at the dealer to plug the key into the computer and see if it is actually working.
I've also taken my glovebox out in preparation for it to not be working and a weekend of investigation, and searching scrappys.
Remote Central Locking control unit 1L0 959 800 is a massive box behind the glovebox.
A Google search shows the same one was in Polos so probably cordoba's etc as well.
Here's hoping I can find summin suitable