leaking overflow and no cold water

Hi all,
We have this problem in our block of flats but cant figure out the problem. its an old victorian conversion with 4 flats. Only the flat on the first floor has no cold water in the bathroom, theyre having to fill the toilet from the kitchen sink. some cold water does flow into the toilet cistern if you turn on the hot water tap in the bathroom! About the same time as as this an overflow pipe started dripping (this is not the toilet overflow). They do have an old water tank (waiting to be removed) in their kitchen cupboard, he assures me this has been emptied as they have a combi boiler in the bedroom for all the heating. The overflow is next to the waste pipe from their kitchen but its impossible to trace the pipe leads from under the communal hallway.
Anyone have a clue what these two problems could be?
thanks in advance
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Perhaps the empty tank in the kitchen is still connected to the bathroom cold taps?
The plumber probably thought that the tank was only used for the hot water, so drained it down and fit the combi onto the mains. However, if the same tank supplied the cold taps, these would now stop working.

This might happen with a mixer tap in the above situation. The mixer probably back feeds the hot water up the empty cold supply pipe until it comes out of the toilet valve.
Christian.
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thanks for the reply,
the tank has been drained for a long time, would this now cause this problem?
also, there isnt a mixer in the bathroom, just two seperate taps.
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If the taps stopped working long after the tank was drained, and the taps are not mixers, then this is unlikely to be the problem.
Christian.
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