problem when flushing Masdee pneu-compact concealed wc cistern; 6 litre?

Installed in 2004 and work fine until this month. flushes ok but instead of allowing cistern to refill water continues to flow into toilet bowl.
If access cover removed and and unit is viewed from above solution is to (by hand) lift the arm on right hand side of cistern; this arm is raised by the black rubber membrane when the flush is operated by the pneumatic push button.

This stops the water flowing straight from cistern and allows cistern to refill. A next push button operation the unit may (or may not) operate ok.
Very tedious and annoying!

Suspect some seal within pneumatic system worn.

Do I need to buy a complete new unit or can existing one be dismantled and problem part replaced?

Also where can I get new unit or replacement parts from?