Ok if the plunger and spring has come out then the valve will need to be replaced.
As there should be a small seal which holds it all together in there and what happens is that seal breaks and it all falls apart.
To fix this you need to replace that valve. Lean it backwards and look inside were the bottle goes and you will see the valve. Its held in by 2 screws. Undo these and then you can remove the valve and swap it over.
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If you need anymore info just get back to me.
Daniel,I have seen your answer and I have ordered the part.RegardsPaul XXXX.
No worries. Let me know how it goes.