| I'd go along with the advice from Mr S.
The preload won't cure the problem. Use the preload, front and rear, to set the sag up. You want the bike to settle about 25mm (ballpark) when you sit on it.
Turn the compression damping up to lessen the dive. It's the one in the bottom of the forks.
If my maths is right, at 125kg, you may need to have the forks and probably the shock resprung and revalved to get the best out of the bike.
Give K-tech (01530 810625) or Kais (01942 896366) a call. | 
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