Usually a flat will be part of a Block Insurance Policy. In fact - for structure, I can't see there being individual policies, as it would entail every proprietor ensuring that others had sufficient cover - and wrangling over which policy has primacy.
Check what the block insurer requires for unoccupied flats (visits every 2/4 weeks etc). Water shut off at main incomer? they may also specify if heating has to be left on at temperature limits.
Local Council will dictate if there is a "nil tax rate" for non-occupied property and the length of time this is allowed (might be 6 months).
You may have to inform the block Factors if you have major works being carted out.
Decide for yourself if you wish to have contents insurance during that period.