Ok, stripped the tank looking for a 1 way valve. (Thought there was somewhere on the line as i had tried to blow it through before... when trying to clear fuel line with compressed air.
Disconnected the line at the tank (line should have been empty as i had drained the tank.
Not a dribble, but no valve. Blew through the line again, squirt of fuel comes out... Blockage in fuel line.

Did actually have strange symptoms when trying to drain the fuel tank through the fuel line...
Started getting bubbles, lots. Thought i must be nearly empty. Kept going, got another 7 litres, had to keep repriming the hand pump due to air. Figured it was empty.
Dropped the tank off and removed the pickup unit. Still lots of fuel sloshing around in there.
Drained out another 7 litres.

Stripped the pickup unit. Found some sediment in there so must be more floating in the tank, ok i could have taken the tank off, cleaned it, flushed it and basically try to get every last floatie bit of gunk out.
Or plan B was to make it more tolerant to junk...
I drilled 6 extra holes to allow fuel into the cup faster, and put some stainless shwarf under the pickup pipe to minimise crap being sucked up, there is a proper filter at the other end anyway...
Reassembled, put 5 litres only in the tank, primed and started, no bubbles.
Bubble problem at least sorted...
Conclusion. pickup Unit is a poor design, any contaminants will block the minute hole that fills up the cup that the fuel pickup draws fuel up from. Then as fuel is drawn from the cup its not replenished fast enough to refill the cup, then it draws air even though there may be 14 + litres of fuel is sloshing round inside the tank...

Now back to my hot start problem, not quite fixed...