Well, being a glutton for punishment I rearranged the run so it goes from the bottom up.

Air is no longer being sucked in from the drippers, but I can't get the flow out of all the drippers. About 2 rows (6 drippers) flow, but as soon as I turn on the next row down the water wont reach the highest row.

Water pressure isn't an issue. If I turn the tap fully on it blows the pipe.

I suspect gravity is the issue Whilst the run isn't that long, gravity is effectively making it significantly longer then it is.

I can therefore either switch to a bigger pipe to run up the wall. Or have multiple 1/4" pipes servicing each row or column of pots.