Hot water tank in loft overflowing & getting hot?

For the last few weeks during the course of the day I have had water trickling out of the white plastic overflow pipe from my loft onto my patio. Now a friend of mine who is a plumber (who I'm always suspect of tucking me up) tells me the trickling water is coming from the hot water tank in the loft and he says the water is hot in the tank which apparantly it shouldn't be. He is suggesting a good clean out with a power flush system at vast expense plus some other work to rectify. Would anyone be able to give me a more impartial opinion as to what's causing the problem. It's an oil fired system. Plus lately the water from the hot taps has been coming through a slight browny colour for the first few seconds when turned on. Thank you.