Radiator warning light on

Hi To you all, I recently bought a 2004 rx8(uk), the previous seller said it had hot start issue, however after running it, the temperature displays midd way ie normal, and starts ok maybe I was lucky as this put off other ebay buyers so got it for 940 gbp, got sat nav boss 6cd interchanger, 18 inch alloys, parking sensors, and even had complete flat tyre original kit. having said all this when checking the fluids all seemed ok bar the coolent level which looked very low, however there was no warning light on, I completely filled it up with fresh coolent,and took for run, the radiator red warning light came on, and wont go out, I have found the information very useful as supplied by other members on this forum, and note that the magnetic sensor can sometimes stick, so I will shake it and see if this works, I also intend to syphon out some fluid and see if the problem is due to overfill, but I cant see why this would matter as there is over spill exit for excess fluid. I have a ford Mondeo there is a mark on the water tank indicating correct fill line, is there a mark on rx8 bottle, I cant see one? I had an rx7 20 years ago, and that had teathing troubles to, guess thats what makes this kind of car very interesting, regards to all.:anger: