With all your bits cleaned and lubricated, replace the piston, mainspring and guide into the cylinder. Place the plug into the cylinder with the spiggot side facing upwards (so it will be on the trigger unit side). Compress the mainspring with your bit of forked tube and locate the plug and secure it with it's nut. Now replace the steel ring into the cylinder before replacing the trigger unit. (You will see the front of the unit should sit between that and the little spiggot of the plug. It seems a crap idea but this one seems better than the G79's I've seen). Locate the cylinder end pin through the cylinder and the trigger unit. Replace the barrel into the breech jaws and locate the pivot pin. Place the end of the cocking arm into the cylinder. The action can now go back into the action or you can replace the sights first. Secure the action in the stock with the 2 front stock bolts and the rear one in front of the trigger guard.
This doesn't seem a bad little gun. It shoots ok, although down in power with the spring I fitted. It does have that crappy "what if" Milbro feel about it though. Like I have said the trigger pins,circlips and screw seem prone to corrosion, but maybe that is just 35 years of neglect? The sights seem out of line--with one side of the rearsight body higher than the other and the foresight is off centre as it just has a screw to hold it and no locating peg as well.