My VW MK1 boils out coolant

I have a VW Golf Chico 1.8 1997 (South African) which i bought from a friend last year that keeps spilling the coolant off the resevoir. If i drive the car at low speeds (heavy traffic or rural roads) it heats up and spills the coolant over and when i open the bonnet(and feel the upper hose) i can hear the sound of it boiling.
I tried to change the thermostat but to my suprise there was no one installed(this suggests that there have been a problem with the cooling system and they decided to remove the thermostat as a short term solution). I fitted the thermostat and the temparature rised almost to the top and the radiator fan kept kicking in. I had no choice at the time but to remove it.
I am suspecting the water pump because when i flashed the system, water was running out fine from the bottom hose of the radiator.
Maybe the water pump is pumping less coolant than its suppose to, which in turn lets the same coolant in the engine for a longer time.
There is no coolant leaking around the water pump but i am highly considering replacing it, is this a good move?

Unfortunately i'm not sure yet how i upload pic but u can view them on my facebook at
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Believe some pumps have plastic impellors and its not unknown for these to come loose.

how hot does it get before it boils? the gauge should get to about 3/4 then the fan cuts in, so check that too. also replace the header tank cap before you do anything else, they are prone to fail and when they do the system cant pressurise so will boil early

Unfortunately the temp gauge goes on and off but when its working it goes up to 3/4 then the fan cuts in, it goes back to 1/2 and stays there but will boil while its standing at 1/2. I noticed a little bit of steam around the cap on one day. The cap becomes very hard to open when the engine is cold. Could this be a sign that its worn-out?

if you see steam leaking out of it i would say yeah its time to replace it. they're dirt cheap anyway

Jonny, i got a new cap and fitted it saturday, drove for about 400 km without any coolant or steam gushing out of the expansion tank. i also decided to put a new thermostat (which was not there anyway) and everything seams to work perfect except for a water flange (the one that houses temperature sensor and connects the main upper hose) which seems to have a hair line crack that opens when the flange is hot. I'm gonna go and buy a new one today and replace it. Thanx a lot for the advices guys.