Oceanis 44cc watertanks

The water tanks on the 44cc are grp moulded between the floor grid and only the top surface is visible with no real inspection hatch. One of my tanks is leaking! Does anyone have experience of this and can suggest how to repair them? Unless there is a "quick cure" the only alternative I can see is to have them cut out and re-moulded, a major job even without the problem of having to remove the furniture! Perhaps there is a "magic potion" that will seal the tank similar say to rad-weld?