To round this one out, I developed what I think is a good solution. My objection to using the same filler for everything is that in many areas it sticks out like a sore thumb. Also a lot of the filler that French Forest used isn't stable and so I hoiked out anything that looked shenzy.
I then used the Fiddes Gel stuff and mixed it with the original sawdust used in the OP. After experimenting with stains (and cocking up..... never a good idea to mix a water based stain with an alcohol based substance...DAMHIKT
) I came to the conclusion that getting the right amount of stain was quite tricky.
So I used some earth pigments and found these much more 'controllable'. Three pots of basic mix and then add Burnt Umber earth pigment in teensy quantities and gradually adding teensy bits until I had three pots of different gradually darkening brown.
It was then a question of liberally applying bits from each pot in a random way (even in the same shake). After letting it dry I found that a fair bit had sunk below the surface to varying degrees. High spots here and there which was just what I wanted. Sanded each area down until you saw the original grain coming through here and there.
Then mixed up more gel as before and reapplied it but using different colours to the original fill at each place.
Keep repeating, the idea being basically building up a random stratum of differing coloured gel fills of varying heights so that when you finally sand it down, you end up with mottled (natural looking ?) finish.
Afraid I didn't do any 'Before' shots.
If opportunity doesn't knock, build a door.