Fluid lying in the bottom of airbox of Vokera Unica HE boiler?

My wife noticed fluid dripping from the base of the boiler on to the work surface below. I removed the cover to see if I could spot a leak. Eventually I removed the cover of the air box and found fluid lying in the bottom of the enclosure apparently seeping down the pipework entry at the bottom and then out of the unit at the bottom. I've dried this off and couldn't spot anything obvious in the air box to account for the fluid. This has only been apparent in the last week or two and then only intermittently. The weather has turned much colder during this period, and I'm not sure if this is significant. Has anyone experienced this problem or have any ideas on the nature of the problem?
Thanks.