When i get home i will give it ago, If i do not get no joy i will let you now thanks for your help.piece of cake to repair, carefully pull the circlip off and push the plunger out making sure the spring is not lost. Change all the o-rings and re-assemble.
I can do this for you if you're not sure about it, or a dive shop may be able to sort it. Depends whether they keep loose o-rings, as most of their o-ring changes are complete kits.