Search Assets in Matrix fields, if they appear in a Section
Managed to make this work, corrected a couple of things and tidied up, but the main issue was realising the field param has to be a string (duh), not a variable. My code now looks like this:

{% set entries = craft.entries.type('artistFolio').limit(null) %}

{% set imagesInUse = craft.assets.relatedTo({
    sourceElement: entries,
    field: 'imageList'
}) %}


This gives me a model with all images appearing inside the Matrix field 'imageList' within the entry type 'artistFolio'. I can then search these as so:

{% set searchTerm = craft.request.getParam('q') %}
{% set results = imagesInUse.search('*' ~ searchTerm ~ '*').order('score') %}

{% for image in results %}
    ...
{ % endfor %}