How can I remove a stubbord office chair gas spring?

Hi

Like many others, my office chair sinks as I sit on it, so, as it's still under guarantee, I contacted the supplier in Germany and they sent me a new gas spring, but I called in the assistance of a plumber friend and he spent ages (glad he only cost me a mug of tea) trying to remove the unit, but with no luck. We followed instructions, looked on You Tube, but neither the tube from the castors, or the gas cylinder itself could be knocked out.

All suggestions will be gratefully received.

Thanks