Not normal to have a hotwater tank in the loft.
Normally there are one or two (coupled) cold water tanks.
If there is another smaller tank then this will be for your central heating radiators and is designed to top up the system if for example you bleed the radiators.
If there is a pipe feeding hot water into this tank then some part of the system is overheating and dumping hot water into this tank
Maurice, November 2010
