Ive had the coil fitted for 10 years (same coil not changed) and just wondered as when I saw a doctor he said as long as it was still working that it was fine to leave in is this correct?

As far as I know the coil needs to be changed every 5 years,other wise its not effective...

There is a slow release hormone in the mirena coil,which lasts for 5 years.

If your doctor says its OK,I'm not going to disagree with a health professional.