Hansgrohe showerselect curious flow issue

[zablermere]

Hansgrohe showerselect shower (push button on/off + temperature) fed from mains/combi. Flow rate dropped. Removed shower head which is fine (tested on another shower). Good pressure and flow rate from hose without shower head. Restricted flow through hose by placing thumb over end - flow dramatically reduced and stays that way even after thumb removed again. Switch off and on again and back to full flow. So seems to be a sticking valve somewhere - the difference in flow rate is too great for it to be a filter. I'm assuming it's either in the thermostatic cartridge, shut-off cartridge or possibly a non-return valve, but is it possible to figure out which without replacing all? TIA

 

[ShaunCorbs]

Would say non return valve I don’t know if there replacements available on that unit