OK so Keg king have told me their standard line length and pressures; so for 5mm line they recommend 3.5m of line at a pressure of around 12-13 PSI (70-80kPa).
I now have a 2nd beer ready and in a keg which I am carbing a lot slower, while still slightly under-carbed (flatish) it poured perfectly last night after 4 days. the other keg is still not. This leads me to believe:
1. I have some form of blockage in the "barb" of the tap (its the only thing I cant clean properly)
2. The beer I made has quite a bit of malt and in general is a very "heady" beer (not sure it is a problem)
3. I have misunderstood the symptoms of overcarbing and as suggested by most people the beer is infact over-carbed.
I will work on those 3 and reportback!
I'd like to thank everyone for their help so far, this forum is a great place to find assistance!!