replacing derwent 3 part syphon diaphragm?

Hi all, thanks for all the great info above.
I have bought a replacement diaphragm from tarpan online for 99p...but i have not figured out how to get the old diaphragm out.
I can take apart the syphon into 3 parts, so i can look into the syphon but not sure how i access the black spring hinge mechanism to take out the old broken diaphragm - any ideas please?