Ross
I feel for you buddy! Is this your first external?
One thing to reassure you with is that Eheim kit is generally very well thought of and well engineered.
If i was in this situation, i would do two things.
1) Strip down the whole thing and start again following the instructions and ensure everything is the way it should be. Taking my time and being very methodical. Using vaseline on all seals that they recommend and checking that they seat properly. Run it under observation and if it leaks move to step two!
2) Take photos, note parts and get in touch directly with eheim. They are always good at their customer service and i am sure they will offer far better advice than the online shop you bought it from.
Stick with it, once sorted it will be plug it in, turn it on and forget!
Andyh