Not a question - but information for anyone similarly caught out.
In SDK 13.0.0 nrf_drv_timer_init() the p_config parameter must not be NULL. In SDK 12.2.0 and earlier NULL gave a default config.
Somehow my timer code, brought over from the earlier SDK, was still working - even with a NULL parameter - in SDK 13.0.0
Or rather it worked - until I added twis / i2c slave support - which casued the timer callback never to be called, but no other issues.
You can try this in the SDK timer example - change the 2nd parameter to NULL, and the exampel still runs.