kamz, the repair care mix is a good product, a bit pricy, but supposed to last.
It's mix goes off depending on the setting time, i think there are about 3 set times to choose from.
If you have large holes, square them off off a bit and use timber hardwood trimmings to fill the holes, bedding it in the mix and allowing a smooth finish of filler over the top. This stuff hold it's shape quite well, then when dry, sand off and paint.
The Ronseal wood hardener, just hardens up the fibres of the wood before filling. Using an alternative filler, then get the 'easy sand' as most of the 2 parts set like rock and you will need an angle grinder to sand it!