Whilst we were away in the new forest, we noticed the water was running out quickly and hot water was so-so. 1x clip securing water pump hose in place broken so fitting not secure and leaking, and leak from behind Cascade panel. Relief valve was leaking and when I attempted to turn it thinking seal might be worn......SNAP......leaving nylon threaded part in the casing. After (a little) cursing, junior hacksaw blade purchased. Gently pushed the blade through the hole in what's left of the valve and slowly sawed a groove along the valve. BEING CAREFUL NOT TO DAMAGE THREADS OF CASING. Did the same again about 90° around. Used a flat blade screwdriver to ease the small section up being careful not to push it into the housing, and removed. Did the same to the rest of the valve thread. Checked casing thread with a bolt I had in the boot of my car left over from fitting of towbar (it was meant to be surplus...), and ready for replacement valve. I do need to get hold of a modified vent panel as the valves are modified and stick out further.
Just waiting for Gary (arcsystems) to return now as all this showed up other fault which hopefully he will be able to advise on.
May not be the proper way of doing the job but it was an emergency solution and I am having to learn the hard way around what my wife calls......our new venture.......hmmmm!