yep, the vent is on the suction side of the pump
gsexr......already been there...thanks.
If it was a new installation, then I'd say it needed an essex flange on the top of the cylinder to draw water from further down and miss any rising air bubbles, but its been fine for 3 years. I'll get the pump off when I get chance and have a ganzy.
Thanks for your input so far.