Depends how close your cordyline is to the drain pipe. Generally it won't cause a problem. However, if the giant rhizome is right up against the drain, it could cause problems.

I'm just wondering how it can be growing over a drain if its planted in soil... The roots on these are not considered invasive, but they do have a fair mass of root structures - the link below might be helpful

https://www.cordyline.org/index.php?option=com_content&task=view&id=42&Itemid=25

Thanks for your replies. I should have said that it is growing over a drain pipe which is about a metre or so deep. As it is now around 6ins in diameter I think I will have to consider taking it out. A pity as it has been in flower this year. I am wondering if it is now too large to consider replanting as it must weigh quite a lot.

Trees with a large tap root such as this, generally do not transplant well.