Not unheard of, but still quite unusual.
This cold water tank in a 1960s bungalow is formed of asbestos cement. Asbestos is a potentially harmful material. Further advice on it can be obtained from www.hpa.org.uk.
The tank should be replaced. A Licences Asbestos Contractor will have to be employed to do the work properly. This will add to your plumber’s estimate.