Ford KA overheating

Being a total novice, I decided to buy a KA (duratec) so I could at least attempt to try a bit of DIY mechanics but I have hit a wall.

The car started to overheat and the coolant was boiling up and the pressure was sending it spilling out of the expansion tank filler cap.

I took it to my local garage and they replaced the seized thermostat - they then told me the waterpump was also not working properly so that has also been replaced.

After a few miles the same thing happened so I bled the coolant as I thought it may be trapped air -no joy. I did some research and as the heater was only blowing COLD air I replaced the heater control valve. As it happens, this was a job which definately in need as two connections were brittle and snapped and it was dry and rusty inside.

Still hasn't solved the problem - still blowing cold through heater (even though the pipes from heater control valve are hot) and still overheating.

Today I flushed the rad and added some bar flush. Left engine running for ten minutes, fan was on for half that time and started to overheat again.

I am being led to believe by a friend that it could be the switch that is causing the heater problem and the head gasket causing the overheating.

If this is the case, that would mean that the thermostat, waterpump, control valve, switch and headgasket have all gone wrong. Is this just my bad luck or could there be a simpler solution?

Sorry this is rather longwinded. I really appreciate any advice.

Many thanks