After looking at internal tank photos on the net, I made the mistake of thinking the barb at the left of the filler cap (to the vent chamber) was a fuel return pipe. I found the description from Laitch, maybe a year ago, was right, after looking inside my tank.
That barb, and the pump filter barb, were 6mm. They were joined by a 6mm hose. The fuel return barb and hose were 8mm. I discarded the 6mm hose as it was softer than liquorice. I replaced the 8mm return hose as it was so brittle that if you even thought about bending the hose, it would crack in anticipation.
If the 6mm hose is missing, it would be an easy mistake to connect the 8mm return hose to the 6mm filter barb, but I would bet my bottom dollar it didn't leave the factory like that.
I spose it doesn't matter why the changes were made, just get what suits your brick.
I will guess the injectors are blocked. There are posts here about cleaning them, and posts for alternative parts. I ended up getting injectors at a car breakers from an 80's BMW.