Just a quick thank you for the information & advice provided here, once again. I have just done the 'spring stretch' and have mains pressure in the van once again.
For those who have never used high pressure hose fittings before (I was one of those until this job needed doing), I recommend searching on YouTube for 'John Guest Plumbing Fittings Usage' and you should find a short video on the RV Service Centre channel that shows how to disconnect & reconnect these fittings. I found it much easier to work on the brass pressure limiting valve once it was disconnected & removed from under our Jayco.