Vokera unica 28HE losing water & pressure?

My parents boiler empties itself, I have refilled it but after about 6 days it gurgles & then empties & the pressure drops to zero.
The plumber who installed it 18 months ago has been back twice to check for leaks but cant find any & says just keep refilling!
He put a plastic bag on the pipes outside, the boiler had been off for 4 days but within 10 minutes of it being refilled there was about 1/2 a pint of water in the bag on the condensing pipe, is this normal?
Is it OK just to carry on refilling?
Is it an incompetant plumber or faulty boiler?

mamaduke
April 2009