2003 Rover 25 head gasket

Hello my head gasket as just stated to a go and was inspected by a mechanic and he said it would cost £500 to £600 to repair I then asked him could I try the products steel seal or k-seal to fix it he said I could try it but I would have to flush out the old oil first before adding the product the car is still running well at the moment but is losing coollent quickly any advice by the way it is a k- series engine 1400 thank you