Large gap between skirting board and wall - filler?

[apsomerfield]

We moved into a flat recently and noticed that there is an area of our lounge where we can clearly hear the family below us. On conversation with the family this is the same for them ie they can hear us clearly in that part too. On further inspection,there is a section with a size-able gap where the skirting board doesn't appear to have followed the wall and there is therefore just the floor board between us and them ie no carpet or underlay etc. Any one have any suggestions over what substance I could use to fill the hole? I imagine it is too big to be filled with a sealant; but wonder if there is some type of insulation people would suggest I might use?

Many thanks

[welsh brickie]

use expanding foam,once its set you can cut of the excess.