I have a 1.3l Ford Fiesta (2000) petrol that has an overheating problem. Heres a bit of background:

Had a major coolant leak through a dodgy drain plug back in December 2011 and didn't realise until the coolant had completely leaked out, causing the engine to overheat. I flushed through and replaced the coolant and this sorted it out.

After a couple of months, the overheating began again, but the coolant level hadn't dropped. Then it seemed to be OK for a few months and it has recently started again.

The first sign of overheating is when the blowers start blasting out intensely hot air, even when set on cold.

The thermostat is OK as when I squeeze the upper radiator pipe, I can hear and see the coolant in the reservoir flowing. The upper pipe becomes very pressurised when the coolant starts to boil. The radiator seems to be really hot at the top, but cold at the bottom and the lower and middle radiator pipes feel cold.

The coolant looks quite dirty, even after only being changed recently and the radiator being flushed out, but there is no sign of any white scum in the coolant and the oil seems clean.
The fan comes on, so that isn't the issue.

I live in Jersey in the Channel Islands, so as such can never drive over 40miles an hour and only ever do short journeys, at most 10miles.
Recently I was driving in the UK and the only time it overheated was when stuck in traffic, but at motorway speeds, everything was fine.

I've given as much info as I can think of, but if you have any more questions, just ask and I will do my best to answer.

Obviously I dread the answer that the HG has gone, but if anyone has any other suggestions, please let me know.

I can do very basic repairs, but I think I will have to take him to the garage to get it fixed, but what I want to know is, what is most likely the problem and how much can I expect to pay for repairs. Bearing in mind, labour costs are on average £55/hr here!