Hello
the filling loop is the flexi pipe to the right hand side.
it is not completely clear from the picture but looks like the yellow handle valve comes of the cold feed and needs to be opened ( in line with pipe )
then when pressure is reached close the yellow valve and also the red handle valve on the same run of pipe from the flexi ( the one that is horizontal )
Ah ha! That seems to have done the job! Thanks a lot. It's (typically!) the one valve I didn't open, because it's the only one that mentioned gas on it