Dave - useful photo to see how the washer pulls up inside. From the image I guess there's a slot on the internal head to hold with a screwdriver while the locking nut is done up onto the rim. With the tyre on, if the valve itself just turns, then I guess its tyre off or perhaps gently using two locked nuts on the valve stem to hold the stem still while tightening the locking nut.
Luckily mine just tightened with no problem. Tyres still fully inflated - just had to try them out on Fri (dry and sunny here if a little cold).
