If your reserve tap is on the right hand side at the rear of the engine, you do not have to remove the air cleaner, but only loosen the bolts,etc and move it upwards until the elbows are vertical. that is the easy part. You should then drain the fuel tank. I preferred to syphon the fuel out as I have found over many years that 30 year old fittings do not like to be disturbed should you drain it from the bottom of the tank.
1. Get someone inside the vehicle to push the reserve knob forward.
2.Loosen the nut securing the cable to the reserve tap arm.
3.Get the person inside the vehicle to pull back the reserve knob freeing it from the tap arm. You must hold onto the nut,etc on the reseve arm otherwise it will drop off and fall, possibly never to be seen again.
4. loosen the silver screw on the brass reserve tap housing and carefully withdraw it.
5.Wriggle out the interior of the tap and replace the O ring, the correct one although circular, is of square cross-section.It can be clicked into place easily if greased.
Re-assembly is as the manual says, reverse one to five above. I forgot to say that you can only do this with your leftarm/hand holding a torch in your right hence the detailed instructions. When you are 18000 km from UK you have to get it right otherwise you are left staring at a perfectly good P6b that cannot be driven!