Rover 200 overheating

I bought the car (216i) about 3 months and have been using it to commute about 70 miles a day. The car had very low milage on it (37,000) for a T reg and the past MOT cert showed it had only done a few thousand miles in the last 4 years. It was running fine for about 2 months and then on one morning it overheated. I topped up the coolant and let it cool down and it was fine again. The same happened a few weeks later. Each time it was after a long run - say 20 miles on a dual carriageway. I would slow down for an exit/roundabout and the blowers would go cold and then shortly afterwards the temperature gauge would start to go up. The car seemed to be keeping its coolant (until it overheated and ****ed out of the reservoir). The inlet pipe to the radiator is always hot but the radiator is cool and the outlet pipe is cold too. Therefore, I thought it could have been a sticky thermostat. I switched this out two weeks ago, bled the system and took it for another spin. Driving around town for about 20mins and all seemed to be good. The next day I took it a bit further and on the dual carriageway. After about 30 miles, I slowed down for roundabout and the same thing happened - cold air through the blowers and overheating. I called the AA and they tested for exhaust gas in the coolant reservoir. They said it was a blown head gasket but I didn't think that the colour had changed much in the indicator. We took the car to a local mechanic who had a quick look over and did a compression test. Couldn't find much wrong with it - a slight misfire in on of the cylinders - he said there could be a bit of water in there. He said if it was a HGF we should try putting some Steel Seal in there and that will sort it out. He also replaced the coolant reservoir cap as he said this can sometime be the issue. I'm no car expert - I only dabble in bit and pieces. The AA man said that exhaust gas was getting into the coolant and causing an airlock which inturn caused the car to overheat and explained why the blowers were going cold. Does this sound plausible? If so, why does it only seem to happen when I slow down after a long run? Does anyone have any ideas? It seems that the k-series has a bad reputation for HGF which leads people to diagnose any issue with overheating to HGF without looking into anything else. I have checked the oil cap and it doesn't look like "mayo".

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