Deed of Appointment

Can I use the mechanism of a deed of appointment to enable the change of ownership (TR1) from mother to son.

Where after divorce father bought mother’s 50% share of house but neglected to server the joint tenancy. Father passes leaving entire estate to son. Is land registry familiar with this approach?

Peter - if you are looking to transfer the legal ownership of a registered title then you would use form TR1 (Transfer) 
A Deed of Appointment might be used for example where a private trust exists as explained in our PG 24
The scenario you describe, if I have understood correctly, is that the registered title is in the joint names of the deceased husband and his ex-wife but not form A (joint ownership) restriction was ever applied for and registered. 
Whilst the shares and severance you refer to relate to the beneficial ownership thy don;t change the legal ownership which has now passed to his ex wife as the surviving joint owner. And if the legal ownership needs to be changed to his son then you need a Transfer, not a Deed of Appointment, by his ex-wife to his son.