Rover never sold the O-ring seperately. It was the whole tap or nothing...Make sure you get one made from fuel proof material.
The easiest way to drain the tank is to disconnect the line from the carby and run a hose into a container large enough to hold what is left in the tank, remembering when full it holds 15 gallons, which is three jerry cans. You can then turn the ignition on and the electric pump will drain the tank. obviously you turn it off between jerry cans...
When undoing the fittings i start with the right angled one using a spanner on the nut and another on the right angle piece. The s pipe fitting is next using the nut opposite for the other spanner and finally undo either the third one while holding the tap or the other end of the pipe (the fuel tap end) by taking the fuel tap off the pump HOWEVER... the spindle is held in by a little slotted screw in the side of the tap, if you can get a screwdriver onto it in situ than you can take the spindle out without undoing all the fittings.
