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 a

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 he
http://ptweb.op.ph.ic.ac.uk/~graceej...%20ceiling.jpg

The cracking ceiling can be seen he
http://ptweb.op.ph.ic.ac.uk/~graceej...%20ceiling.jpg

The top coating of the ceiling then raises up:
http://ptweb.op.ph.ic.ac.uk/~graceej...%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...%20closeup.jpg

Use a moisture meter to prove water is not the issue.