I was lucky enough to find the correct tool a few years back, they are fairly tight and need knocking back on. I use a scaffold pole for that
http://www.commervanspares.co.uk/
http://www.yorkshireclassiccampers.co.uk/
http://www.commervanspares.co.uk/
http://www.yorkshireclassiccampers.co.uk/