What the heck is 12.5 stone, 175lbs or so? Is that with gear on or without?
If it's without gear on then you're prolly around 190 with gear on so the good news is that you don't need new springs. Get a friend to help you set the sag to 30mm, then put compression and rebound on both the forks and the shock in the middle of the range and dial out or in as desired. You're going to need a ball ended hex key to access the rebound on the rear shock but any decent hardware store will have one.
Also don't forget to set your tire pressures. Too high and the bike will feel numb and you'll wear tires (not to mention traction will be iffy). Too low and the bike will feel sloppy. I've worked my cold pressure out at 30psi rear 33psi front given that ambient temperature is between 20-23 degrees (aggressive street).
Most important thing to do is to set your sag correctly and go from there.