Older cold water storage tanks in lofts were sometimes made of asbestos cement, and HSE lists water tanks among common locations in pre-2000 buildings (HSE). They are usually grey, heavy and rigid, unlike modern plastic tanks.
Your options
- Leave it if it is sound and no longer causing problems. Many are left in place when a new tank is installed.
- Have it tested before any plumbing work that would cut or move it.
- Have it removed whole by a competent contractor. Asbestos waste is hazardous waste and must go to a permitted site with a registered waste carrier.