Moto Grand Prix
Main Motorcycle: 1973 Triumph TR7RV
Join Date: Oct 2013
Location: Pleasant Hill, CA USA
Other Motorcycle: 1964 Triumph Mountain Cub
Hi Ian, I would just start with removing clutch plates & measure stack height. That will only take about extra 1/2 hour. Then you'll not be guessing on spring tension.
You will then know the next step. If notching is acceptable, and spring tension was not tight enough, you know correcting spring tension is reasonable next step & would be expected to correct slip.
If tension was already tight enough or notching is excessive, you know you'll need to do a full tear down & repairs.
Doing another guess on spring adjustment is not productive way to go.
Also, please take very clear detailed photos of grooves on basket & hub while plates are out. That will give us an idea of what you have & be able to make a recommendation.