Okay, so in case this is of use to anyone else. The original checkbox hack tut that I was following came from here.
I’ve got around the issue that I was having in WordPress by wrapping my checkboxes in the labels like
<label><input id="fieldjtdktry-0" name="cm-fo-jtdktry" value="4469820" type="checkbox" /><span></span>Learn</label>
and then just tweaking the CSS selector to read
label input[type="checkbox"] + span {
instead of the original
input[type="checkbox"] + label span {
Works like a charm!