Boiler pressure dropping immediately?

We have just come to fire up out combi boiler due to the cold weather. Everything was ok but the pressure suddenly dropped with lots of water flowing out through the condensate trap.

Now whenever I test it the water pressure drops pretty much straight away and I get a flame failure. All the water seems to flow out the condensate trap straight away.

Is this a "get a man" in problem or something obvious I could fix myself? The rads have all been bled and seem ok.

thanks