Ok just to close this thread. I removed the rear dampers today, put a jack under the chassis and set the ride height to 135mm. I then measured the void where the damper should go, bolt hole-to-bolt hole, 350mm. I have the Mk1 chassis and it will allow for a greater ride height than this before the wishbones hit the chassis rail.
So yeah, front dampers 330mm open, Rears 350mm (or more) open. It would seem that Marlin did indeed supply me with four front dampers. My fault for leaving it so long before checking!
Thankfully Alex200mph can help me out with a set of four Protech dampers and shocks of the correct size. Phew