Potterton E133?

I was in the loft and noticed that the flue to my Potterton HE24 had come apart at a 90 degree joint. I pressed the plastic pipe back together (this has happened once before). AS I did this I heard a trickle of water flowing backwards towards the boiler.

The boiler started to flash E133. I noticed that the inside of the boiler was awash with water (condensate?). I know E133 is a gas flow problem, but any idea on what actually may be broken?