How should I fill these holes?

[chuckey]

If you can stuff newspaper down the crack first, saves trying to fill the whole void behind the skirting board. I go for ready mixed filler, I find Polyfilla too soft so when you sand the item, where the Polyfilla is there will be a depression. Conversely I find Polyfilla fine grain too hard and "gluey", when rubbing down, always leaves a small mound.
Frank