What's the best way to fill this gap so it can be decorated to match the skirting? Anything easier than cutting some wood to fit and using caulk/wood filler over the smaller gaps left?
I'd do what you suggested but use wood filler to blend in to the upper skirting section, as its quite a big gal. Then caulk for any last bits if needed. Filler has the advantage of being able to be sanded.
You probably want a fractional space to allow for any wood expansion /contraction.
What's the best way to fill this gap so it can be decorated to match the skirting? Anything easier than cutting some wood to fit and using caulk/wood filler over the smaller gaps left?