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?