Mg zr 1.8t over heating after k-seal

My radiator has got a slight leak and every 2 weeks I need to top up the water. So the weekend I put some k-seal in to try and sort the leak temporarily. I then drove the car on the night and after 10/15 mins of driving noticed the fan had come on and the temp gauge was slowly creeping up. It hit the 3 quarter mark and then it went back down to it's normal position (just under half way) it then kept doing this until I got it back home. Does anyone know what might have happened? Could the k-seal have blocked the rad or messed up the thermostat? Any help will be appreciated.

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