Your general idea is right and possible but you have to check if your mainboard offers an option to shutdown the PC if the rpm signal is not present anymore.
Ok thanks for the information. I have just tested this and it would appear that my motherboard (in fact all new UEFI bios motherboards) do not have a shutdown procedure built in to the bios upon reading a fan failure.
What are my other options please? Do i need to purchase additional items from you to make this happen?