This is an EASY one to solve.
Dont do as I did and take all the floors out to get the gearbox tunnel out and then find there was no need.
I will bet 90% that it is a tiny grub screw where the gear lever meets the gear box. It is on the side and is prob about a 2-3mm allen key size tiny tiny bolt that has vibrated out. I took all my gear lever out and took it to a garage and said why did you bring all this in, all you need is 1 of these. The screw cant be much longer then 5-7mm and as i said it has a allen key head on it.
If you have a rubber sleeve over the bottom of the gear lever it is under there.