This morning the hatch only opened to the point at which the strut pushes it up (about 20cm)
I tried a couple of times so I have replaced the lower 2mm rubber washer with the same as the top one. Now both are 3mm
It now opens fully every time
post note; I found that after a week or so the bottom rubber washer became loose and allowed the spring to slip a bit.
I have solved that by replacing the bottom rubber washer with a Neopreen cone washer taken off a high end tap washer ($2.95)
As it is cone shaped, the smaller end fits the raised lip of the gas strut body perfectly and the larger end fits the metal washer under the spring. It was just a case of prying the Neopreen cone washer off the tap washer body and drilling it out slightly.
Have checked several times a day for the past 5 days and the system works perfectly.
I hope this helps anyone else wanting to fit the springs etc