So I've rebuilt the seat and tried to show how much deflection I'm getting with the cushion and just 2 springs fitted uisng my charming assistant who claims to be a mere 70Kg (and who am I to argue).
Here is the rebuilt seat....I'd replaced the backrest cover a year ago (holes in the bolster) and now the new cushion matches:
This is the underside after the rebuild with just 2 springs:
Here is a shot taken from the rear of the seat with no weight on it:
And here we can see the deflection with the '70Kg'
Now I wish I'd taken shots underneath before I started! My 'seat of the pants' impression is that the deflection with just 2 springs is quite marked but I'll have a better idea once it's back in the car. I did a clearance measurement 'top of head to hood' before I started and should be able to get a reasonable idea of the difference.
Of course like MGRover I now need to drive around California and Nevada for a few months to test durability!
Richard
1.8i Mk2 Solar Red, 16 inch square spoke wheels, MGFMania hood with zip-in glass rear window, DRLs, Kmaps ECU, Pipercross panel air filter, MGOC Supersports back box & some cockpit bling