Not cool runnings!!!

Hi all, I know this has been a well discussed topic but having read all the threads I can and having done almost all things I am well and truely buggered .
My wife and I have a 1983 1.5 (1457cc) petrol cab, with the ignition on we get the overheating warning light (with the others) as is the norm and the car starts and runs without a problem. Every time we drive it the temp gauge goes straight through the mid range and sits at the bottom of the red yet we never get the warning light coming on, however when we have pulled up we get steam coming from the expansion cap on the radiator as we have no expansion tank, also the fan cuts in and runs as it should also.
I have since changed the thermostat and flushed both the radiator and the block, when refilling the cooling system I added 1ltr of antifreeze and 3 ltrs of water. There are no visible external leaks and the car still starts and runs fine, having felt the lower water pipe from the radiator as suggested it is clear the new thermostat is operating correctly, however we are still getting the same problem with running hot and steam from the expansion cap.
As this is our first mk1 golf there are a few things I am unsure of!! From the different websites I have looked at for parts should this car have an expansion tank? the blue expansion cap, ours is black and all I can find are black ones and not modified blue ones? as the car had only done 140 miles in the year before we bought it could this have an effect? The car also seems to be running rich, if this is the case could this be causing the engine to run hot?

Any help would be great as this has become all too confusing!!

To get the ball rolling, I'm pretty sure you should have an expansion tank on a 1983 engine.
I know you can find the blue caps on Ebay : http://shop.ebay.co.uk/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=mk1+golf+expansion+tank+cap&_sacat=See-All-Categories

Mine is a 1457 cc 1983 and does not have an expansion tank. It has a top fill rad. Get a cap from VW, its cheap too