The seals that Caterham installed are junk! I had smoke from my car when starting and lifting. I pulled the head and replaced the valve seals and that cured the problem. It is easy to replace the seals and check they are the problem.
Getting the correct seals is a real issue, there are two thicknesses of valve steams and guides and you have to get the correct ones. I needed the narrow ones, with valve stems The ones I ordered from Burton Power are SKU: FP705A "Valve stem oil seal (each) - suitable for duplex valve springs: Ford X/Flow OHV Kent, pre-X/Flow & BDA FP705A". The critical dimension is 10.57 mm diameter on the valve guide.
Be careful if you try to remove the valves from the head. They will probably not glide through the guides because the spring retainers will have expanded the slotted part of the valve stem. You will need to use emory paper to sand them down to a diameter that allows them to glide easily through the guides.
Do yourself a favor and order from Burton power. I have 5 sets of incorrect seals that I ordered from other vendors!
Good luck/Anker