On another note I see that you can buy a K&N induction kit for the 100HP. A mate of mine told me that some filters (like the K&N) can gunk up the air intake system with a fine oil. He said it would be a good idea to fit an oil catchment tank if these filters were used. Not sure if the BMC filters have an oil coating. My mate does know his stuff but im not sure how acurate the information is.
I have checked the GSR website and their induction kit is cheaper (£177) than the BMC cda. When you say they dont come up for sale often do you mean that GSR have ceased trading? I am no expert but I cant see what is so special about the GSR unit. It sits very low in the engine bay though I wouldnt be very keen on that idea.