i've just got the Fallout New Vegas Ultimate Edition, loaded it up, and cannot assign anything to the number 2 as a hotkey.There is already an icon in there but I've no idea what it is. I can't override it so I've tried dropping all my weapons then picking them up again but it's not working.

Hotkey 2 is used for ammo swapping on the player's currently equipped weapon in Fallout New Vegas.
I hope this helps.

That explains it, thanks. Any way to remove this? I'm planning on melee/ unarmed so won't need ammo much

theres no "real" way of changing the 2 key from ammo swap