Just putting this here in case anyone else comes across this issue, esb_arm.lib and gzll_arm.lib don't appear to implement the work-around in PAN 18(in the version I tested) resulting in higher than expected idle current. Not sure if it's intended for the libraries to implement the PAN workarounds or not so putting this here rather than as a bug report.
Adding NRF_RADIO->TASKS_RSSISTOP = 1;
to the tx_success and tx_fail callbacks resolves the issue.