Firstly have you noticed more cracks and splits since adding central heating? Central heating dries the air out and removes moisture from wood and plaster so they shrink and start to show cracks/splits.
Rake /open the cracks [you'll waste your time trying to fill hairline cracks.] then apply water based wood glue, purchased from your local merchant, to the cracks. wipe the excess from the surface.
When the glue gets tacky apply the filler. let it set for at least one day then sand down. Unfortunately central heating plays havoc with plaster and woodwork causing dry air. Some say leave small dishes of water in each room most effected and it helps to keep moisture in the air.
A tip from a carpenter drill a hole in the end of split in wood slightly bigger than the split. This stops the split 'Travelling' then fill both the hole and split.
BOB
February 2005