Hi there Sean and Welcome to the forum
Basically the syncro's are worn in your gearbox and it needs to be rebuilt.
The longer you leave this the mote damage is caused to the gearbox but Also to the Engine!!! As the classic Mini uses the same oil for the engine and gearbox, the shards of metal from the worn syncro rings will go up into the engine and do damage sadly
Where are you based?
There is an excellent Mini and MINI specialist here in Cork who do everything Mini and also can rebuild your gearbox.
Ger from O'Leary's Garage is the contact on 086-8663828
Fergal