There are a few methods you could use Greg.
Fixing to the inside skin will be sufficient.
Either spag.into the ends of the bricks or timber wedges in the mortar joint. 3 or better still 4 either side.
An alternative although harder to execute is to toothe out the mortar joints both sides, fit brick ties to the jamb & slide it in. Inject chemset or similar & then repoint the joint on the internal face.