Its definately not the seatbelt buckle. It is the back left side of the seat i.e. under your left buttock. It doesn't just squeek as you go over bumps etc it squeeks all the time even as you move your leg to depress the clutch.
It is an electrically heated seat with electric adjustment, if I can find a way into the seat without removing the upholstry am I alright to spray WD40 inside the seat?