Ford Focus over heating

hi. i have a ford focus 1.6 2000. 2 days ago i thought i would put in a product called k-seal in the top up tank just in case i ever had a leak in my water pump or pipes after shaking the bottle for 30secs i put in the tank but there was some left which was really thick i got it out and put the cap back on the tank and went for a drive so it would work through the pipes. 2 days after my wife and i went for a drive and was doing 60mph and i had to overtake a lorry and i was doing 70mph and the temp gauge went on to red so i slowed down turned the heater on full and it went back to normal so i done it again and it went back onto red again so i slowed to 50mph and went back home and took the car to a garage and was told it was the head gasket and was given a price from £600 to £800. which i cant aford that much. so what is bugging me is the car was ok untill i put this k-seal in i could drive 70mph or up to 80mph and everything was ok. i dont know what to do now can anyone give me some advice. thanks david