Thanks for the assistance guys.

Its a 2006 model with 40000km on the clock. The water does not boil from the radiator cap, but from the over flow bottle next to the radiator. Unfortunately i havent had the time to check the sensor and its resistance or do a pressure test, which will probably be the most conclusive as to where the problem is, and if the cap is in fact sealing 100%, but will try and get to that in the next day or two.

Im pondering whether I should connect a resistor eg 100ohm at the sensor to enable the fan to switch on a tad earlier than normal...but i guess Yamaha made it that way for a reason.

Thanks again for the advice, its much appreciated .