I'm sorry to say they do them up so tightly that they can't even undo them themselves without causing damage. You should see how mine came back from a service.
You will need two lengths of wood about 400mm long and about 50mm square or thereabouts. Then you will need a router to cut a hemispherical groove along the two lengths, like inletting a stock. Then you line the grooves with something like old slices of rubber glove, place them around the cylinder and clamp the lot in a workmate.
That should grip the cylinder, now all you have to do (!) is to undo the reg. I bet you a hundred Rand the C spanner slips or damages the reg outer casing.
I designed up a sleeve like an old hex nut from a truck wheel, bored it out to a close fit to the reg housing, drilled three holes at 120 degrees and put silver steel pins in it to engage with the holes in the reg body. If you work out how much torque you can apply to this without shearing the silver steel pins, that ought to get it undone. You won't get all three taking load at the same time, two will have to give slightly before the third one does any work.
In point of fact I didn't get to use this tool, as I found someone here who could do the whole job for me.
If and when you get it undone, there should be no need to do it up so tight again, it's an O ring that causes the seal.
If you get left and right drift of POI with change of reservoir pressure like I did then float the barrel.