I want to check that the reason my heater doesn't heat is the possibility of an air lock in the system. I know that in order to fix this I need to disconnect the matrix hose but despite looking in both Discovery service manuel and Haynes book I'm no wiser as to wear this hose is!

Can anybody guide me to this holy grail of heating?

The hoses are at the back of the engine...

follow the two pipes which run along the top of the injun, back towards the bulkhead. yu will find the pipes there.