John
Thanks so much for your detailed and helpful reply. Lots to unpack here.
You're absolutely right - all the old parts were thrown out at time of doing the job and I've probably learned a lesson there.
I'm fairly skilled with the chisel as I have already had to retrofit 5 other 'standard' door in the house and have found that some good wood filler and paint can go a long way to make up for any minor indiscretions.
I'm going to work through the process you've described but this may not be the last you hear from me.
Cheers buddy