Hi @rosebarton,
Thank you for your question and welcome to the Bunnings Workshop community, it is fabulous to have you with us.
I don't think there is any need to replace the jamb, however, it would be best that you chip out the filler and start again.
I'd suggest you start by unscrewing the striker plates from the jamb and temporarily removing them.
You can then use a chisel to chip out and remove the old filler. The chisel should do most of the work, but it is worth giving everything a quick sand with some 120 grit sandpaper to remove any remaining filler and prepare the timber for the new filler.
You should then clean up the striker plates. This can likely be done with a wet cloth and some elbow grease, but you may need to wet sand them with 1200 grit wet and dry sandpaper if the old filler is particularly stubborn.
Once cleaned up, completely cover them with masking tape before screwing them back into the same holes they were taken out of. If the screws are not biting well, then you can take some toothpicks, stick them in the screw holes and snap them off. This will give the screw more material to bite into and help anchor them in place.
You can then use a 50mm plastic scraper to apply builders bog where required. Wait for the bog to dry and then sand with the 120 grit to get it nice and smooth before painting to match the jamb.
Once painted, remove the striker plates, remove the tape and then reinstall.
This will give you a nice clean repair that if done well will be imperceptible to the eye.
Let me know what you think and if you have any further questions, please don't hesitate to ask.
Jacob
