Strange blistering ceiling - leaks?

The flat we are in has developed strange blisters on the ceiling. They appear to be the result of material in the structure of the ceiling expanding and breaking the surface.
Above the ceiling is a flat roof, however there is no obvious sign of water damage or dampness around the regions of damage.
My questions to the community are:
1) What do you think it is caused by?
2) What should be done to check?
3) Apart from filling and repainting, what could be done to avoid this?
An overview of the ceiling can be seen here:
http://ptweb.op.ph.ic.ac.uk/~graceej/tmp/Overview%20of%20ceiling.jpg
The cracking ceiling can be seen here:
http://ptweb.op.ph.ic.ac.uk/~graceej/tmp/Cracking%20ceiling.jpg
The top coating of the ceiling then raises up:
http://ptweb.op.ph.ic.ac.uk/~graceej/tmp/Raised%20coating.jpg
Finally the coating can separate completely, revealing the material in the ceiling:
It appears that there is something forcing its way through the concrete....
http://ptweb.op.ph.ic.ac.uk/~graceej/tmp/Hole%20closeup.jpg
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Use a moisture meter to prove water is not the issue.
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