Victorian Terrace Persistent Minor Roof Leak

[88v8]

And given that it's only single brick, with lead under the capstones there will be less cement bond to hold them in place.

This is a good point - it could exacerbate what is potentially already the cause of the issue.

I would renew the bricks. Use engineering bricks, no additional lead.
Engineering bricks are not absorbent.

I assume we're only talking of replacing the parapet bricks (i.e. the top course only) with engineering bricks, and not any lower?

Yes.
Just the top courses that are exposed to the weather.

Ivor