Dudsy is right - should be taking the head of in a hurry, at it's coldest point the flexible hose is not so flexible and easy to damage.
Expert board are a bit of a funny one, what I found helped me was instead of using seal string on the back plate- I used a piece of 3mm thick neoprene, cut about 5mm larger than the backplate itself and used that instead of seal string.
I also used the same neoprene to fill the area between around the cpu socket and the mach kit - filling the air gaps reduces the risk of condensation also.
The next step is to use the neoprene as a gasket for the head too, not essential but allows easier removal and replacement of the head if required.
I did use old seal string to cover the exposed electrical parts found inside the perimeter of the CPU kit (around the socket) for added protection.
The front plastic as well as the first lot of seal string I then put a seal around the outside of the plastic (like a seal used around teh edges of kitchen surfaces) for added protection.
Sounds like a lot of work, it's not and it's better to be safe than sorry.