SIAMP inlet valves, I hate them!

They are so finiky to repair, had another one tonight on a closed back toilet, constantly filling. diaphram split to fuck so swapped it and turned the water back on, ran upstairs and its still not shutting off, fiddled around but still not shutting off. Had a spare one in my van so robbed the complete arm assembly off it and tried that, same shit so back off with the water, tried to unscrew the valve from its stem but I can't get a turn on it and its tight in the cistern corner, it now gone 7pm and I'm thinking I'm going to have to drag this whole bastard bog out to swap the  thing for a viva or fluidmaster then I went looking in my van again and found a second siamp valve exactly the same so I used the arm off that and it worked! two indentical parts but only one worked.

I am aware of the small clear washer that also needs to be swapped or turned over hence replacing the whole arm assembly.