Conventional boilers have water tanks in the loft - one to supply the central heating and another to feed the hot water storage tank.
Modern Combination (combi) boilers do not need these tanks.
If "your" tank was in a neighbour's loft how can you be sure - and lay claim to it being your tank.
If your hot water and central heating are both working OK then (a) you have a combi boiler and (b) there is no safety issue.
Things are left unsaid in this question. Who paid the plumber?
Peccavi
November 2010