Clogged J-Head

Hi guys, I have a bit of an issue. I assembled my Prusa i3 kit from 3dprinterczar.com and I have to say, the kit was great, not too much problem solving. Everything is working fine apart from extrusion. I've managed to get it to extrude maybe 20mm of plastic, but the thing keeps jamming solid. I've had to pull it to bits, pull out the plastic tube thing, and clean it out over a gas stove-top and an air compressor like 4 times now, and each and every time it works for a very short period of time. The pressure also seems low.

Before I go any further however, I think I may have identified the problem, but I have no idea and it's not something I can easily identify over the internet without asking.

The small peice of hard white tube inside the J-Heads heat-sinked part is probably ~3mm in diameter. This printer is meant to come with an extruder that does 1.75mm plastic, and of course I've only got 1.75mm ABS. Is the plastic tube meant to be also 1.75mm? Because every time it blocks up, plastic back-fills this white tubing, swells, and I suspect this is not only causing the blockage, but also the low pressure when it does work for the first few seconds because while it's extruding, it's also filling the space in the tube.

So yeah, I have no clue. I think I'm going to order a new 1.75mm j-head from here in Australia. :s

Thanks in advance for any help.

Edit: Here's a picture of the inner-tube. The ABS has kind of come back out and bent back over, but each and every time it blocks, it has looked like this. (Sorry, it's difficult to see)
https://www.dropbox.com/s/zzosvqdtz8...72519.jpg?dl=0