When assembling both the front and rear suspension, a quick comparison of the dampers showed that they were not set to any particular position. Caterham also confirmed that they would set the ride height accurately, and allowing for my weight, during PBC. However, I wanted to at least set them roughly.
I was unable to confirm the exact sizes of C-Spanner required for the front and rear dampers, even when I consulted Bilstein directly (they promised to email, but never did). Having read about others using a ground down screwdriver or pin punches, I measured the diameter of the holes in the adjuster ring and found a good comparison with the diameter of a redundant stubby screwdriver. A few minutes on the grinding wheel and it looked like a reasonable solution.
Using a redundant stubby to adjust the dampers
The process I used was first to measure the existing ride height of the front and back suspension (from the ground to front chassis tube and ground to A-Frame mounting point respectively). Both measurements were well below the recommended distances of 150mm and 165mm respectively.
The lower ring was then wound down to the limit of it's travel on each damper to act as a reference point. Focussing first on the front damper, I then measured the distance between the upper ring and the lower ring with vernier callipers and then made adjustments to ensure that, in the first instance the front dampers were both the same. Then it is a process of trial and error by adjusting both dampers an equal amount and measuring the resulting ride height. Adjusting the dampers is considerably easier when they are not supporting the weight of the car, and so this was a fairly time consuming exercise of raising the car, adjusting, lowering the car and measuring. Eventually I got the car ride height to approximately the correct height and gained some experience in the procedure. Caterham confirmed they would accurately set the ride height, including some compensation for my weight in the driver's seat, as part of the post-build check.