Got there in the end. Like others 23d was my LOI, for the longest time I though it had something to do with taxes and taxis! I got it eventually! Still not sure I completely follow the reference in 8d
Geeker's hint is really helpful - but if only single digits are included, I can't see that there's an exception. They're all there. (Assuming that we are counting those in answers like 4a as well as those set out like 5d and 18d.)