We have a C-shaped brick enclosure (back and two sides) which is about 3
feet high. It was probably once used as a compost heap.

We want to enclose it by adding a fourth wall, to make a raised flower bed.
It so happens that we have some spare bricks, and can buy any additional
ones if we need them.

What is the best way of tying in the new wall to the ends of the two side
walls? Is the best way to drill holes in the existing mortar/bricks, mortar
in pegs and then lay the new bricks so the pegs fit between the courses of
new bricks in the same way that butterfly ties are used?

https://s11.postimg.org/v5qv1gnar/enclosure.png

I'm reluctant to start chiselling-out the old mortar to make room for proper
butterfly ties, in case the mechanical shock fractures the mortar elsewhere.

It's only a low wall (probably about 10 rows of bricks) and the only load
will be from the soil inside the enclosure. The bricks would be standing on
a concrete base, not on bare earth, so they would be unlikely to settle over
time.