Hi Alasdair,
In this case, lilypond is trying to center the instrument name on the same line as the composer name. As they are both long they do not fit, and consequently overlap.
You can make a custom header, and probably put the instrument name on a separate line.
Refer to the Notation Reference (affectionately called the NR around here) Section 3.2.2. Custom titles headers and footers.
The MWE enables us to solve it quickly!
Andrew