Picking up this thread again.
Having originally spoken at length to GSE ( thanks Gav for the tip ), I held fire on getting replacements as I wasn't totally happy with the "OPEN" height of the tailgate with the OEM struts, and I thought that a simple replacement wouldn't quite achieve what I wanted.
Having removed both of the struts ( I hadn't appreciated how heavy the tailgate actually is ), it is able to open much higher than the struts allow, as the door hinges do not "lock-out". This got me thinking.
I did quite a bit of playing about and measuring with the RTT on the rack, to get an idea of what would work; when in use, the RTT cover unzips and lays on top of the tailgate in its open position. One of the minor irritations, besides the poor opening, was that if the RTT was open and the tailgate was up, the extra weight of the tent cover caused the gate to sag/close slowly, with the inevitable need to constantly push the gate back up.
The old OEM struts were measured and are charged at 300n (i.e. able to exert 67 ftlbs force) They cannot be recharged as they can't be taken apart. Rather than just increasing pressure, I decided to get slightly longer struts; this will allow the gate to open a little higher. The new struts will be charged to initially 400n, (90 ftlbs). This should be enough as it's an increase from 600n to 800n combined. However as the new struts can be dismantled, they can be charged to up to 600n (135ftlbs) if necessary
GSE, have been really amazing in dealing with this for me. They have taken the original struts, have drilled out the OEM brackets and fitted them (with new hardware) to the new struts. For such a small job they couldn't had been more accommodating and helpful... what a refreshing change to deal with a business with that attitude.
I will collect the new struts tomorrow and post pics of them against the OEM version, and the height of the gate when open.
Will keep you posted and hope that my decision to go for longer struts doesn't backfire and I end up with them not fitting, unable to close the gate or it opens too far and high!