Some of you know I have fitted new seats to the standard runner’s.
Been for a long drive today and 5 times under hard acceleration, the seat shoots back, leaving me dangling for the gear stick and throttle.
I can get it to do it if I push hard against the steering wheel.
What have I done wrong when refitting the seating and attaching the runners to the new seats?
This happened to a friend who is not very tall when I let him drive my Caterham. He left the pit lane, the seat went backwards and he could not reach the brake for Redgate. I recommend he got taller as a fix.
It must be the spring loaded position leaver is not locking into one of the adjustment holes. Maybe when you fitted the seat to the frame it twisted so the parts do not line up.
Seat removed from the car, taken off the frame.
All seat bolts put back loose then gradually tightened. Runner’s move freely when using the bar, seat moves fine and everything is lined up. Have even used a tape measure to check runner’s move at the same length.
Who assembly put back into the car. Front two bolts fitted loosely, bolt next to centre console fitted loosely but for the life of me, I cannot get the final rear bolt to line up with the hole in the chassis. 
Are you using the right set of holes? There are two sets, presumably for RHD/LHD variants. You need to use the inboard most holes at the rear iirc.
Yep, it’s the correct holes.
Seat is back in now and secure. I loosened the seat side bolster bolts, as these gave the few mm movement I needed to get the runner’s to the chassis, then re-tightened the bolster bolts.
I have pushed back and forth as hard as I can, bashed myself hard against the seat back and it hasn’t moved, so hopefully it’s sorted now.
Comedy photo at the request of Andy Bond
This wasn’t the shot I requested!
Glad you are sorted!
It’s hard to take a photo when your head is stuck between the seat and harness bar 
