Condensate leak on flue?

I noticed water dripping fron my Glow worm 38cxi at the weekend. Taking the inner casing panel off there was a fair amount of water in the tray below the combustion chamber.
At first it looked like the condesate siphon had stuck, so I removed it and cleaned it. Checked later and more water.
After much searching around I found that it was indeed condensate but it was dripping from the 90 deg elbow on top of the boiler and running down inside the case.
The rubber sealing ring (push fit plastic flue duct) on the flue elbow appears damaged.

Can this seal be purchased on its own or am I looking at a new flue system (Boiler not under warranty)

And is this a common problem.