just 2 'O' rings and filter basket per injector. also a bit of vaseline smeared on the orings makes it easy to put in and less risk of twist seal causeing leak.

also another test for a air leak is have the eingine running and spray carby cleaner around the gaskets on the inlet side if the revs change at all u have a air leak, which cause also feel like a miss firing or fuel surging

Thanks for all the tips and advice. As my vehicle has done in excess of 300 k i am going to treat it to a new set of injectors. Petroject can supply set of new Bosch for around $270 which sounds fair. Will flush out all lines at the same time and fit (another) new fuel filter.
Cheers