This is a Overheat problem Blasting with Air only blows it further in to the Laptop, it needs the top bit of above the Keyboard to get at the CPU Heat Exchanger it will be blocked with what looks like felt, its Dust and hair etc. If your a Smoker it will make matters worse
Many thanks for all these suggestions. The fan is clean and I've checked the power options - it still runs hot. The fan comes on almost immediately I boot up which says to me it's too early to be heat - so what else can be causing the fan to constantly run?