First of all, you should establish where is the leak coming from. It could be due to the plumbing fittings that are not water tight, etc. This could be easily solve.
If the leak is due to the age or cracking of the water tank itself, then you should have it replaced.
For long term, better to replace the water tank if it is an old tank. Since you are still in renovation stage, better to have the inconvenience now then later when it leaks again.
Believe your house is a link house. Try to see if there are alternative place like a flat roof to place a new tank if it is difficult to open the roof to replace the existing water tank. Else, just pay you contractor to replace the tank. He may need to open up the roof.
Since you are at it, might as well take the opportunity to enlarge the tank size to increase the storage capacity. In case of water cut.

If it is a new house, it is still under DLP? Then can get the developer to replace the leaking tank... all cost borne by them

If it is not the leakage from the water tank, then the water mark could be due to the water when your plumber was making some plumbing connections, etc.
Else, could be from the roof.