Sticking valve on Gazco fire

[splashy]

Help/ assistance required, please.
I have a Gazco coal effect gas fire in an open grate, fitted around seven years ago.
There were no obvious faults with the fire at the time, but due to its age I decided to have a service done last October, and responded to a flyer put through my door by a local gas fitter who used to be employed by the firm which originally fitted the fire. He did the service, left the fire on and departed. When I went to turn it off that night I found that the valve was very hot ( hotter than I ever recalled it being before) and that several hot coals had fallen under the fire. I couldnt shift the valve--it was stuck fast-- and had to turn it off at the meter. To cut a long story short, after some discussion as to whether the valve needed replacing and on my insistence that the valve had been fine before the service, eventually the gasman informed me that the valve needed greasing. This he duly did, and it was fine during last winter, but having started using it again its become very difficult to turn off after being on for a few hours.
Any advice as to how to proceed with lubricating the valve would be much appreciated, bearing in mind that I'm not a gas fitter, and therefore not qualified to work on the supply itself.