With this fix it seems more reliable but it did crash again last night leaving a veye_raspistill Zombie. Short stack trace:-
Apr 11 23:31:21 allsky kernel: [22053.109872] WARNING: CPU: 3 PID: 20358 at drivers/firmware/raspberrypi.c:64 rpi_firmware_transaction+0xec/0x128
Apr 11 23:31:21 allsky kernel: [22053.109884] Firmware transaction timeout
Apr 11 23:31:21 allsky kernel: [22053.109895] Modules linked in: rpcsec_gss_krb5 8021q garp stp llc spidev brcmfmac brcmutil sha256_generic i2c_mux_pinctrl i2c_mux raspberrypi_hwmon cfg80211 rfkill bcm2835_codec(C) v4l2_mem2mem bcm2835_v4l2(C) bcm2835_isp(C) bcm2835_mmal_vchiq(C) videobuf2_vmalloc snd_bcm2835(C) videobuf2_dma_contig videobuf2_memops videobuf2_v4l2 videobuf2_common snd_pcm snd_timer i2c_bcm2835 snd spi_bcm2835 videodev mc vc_sm_cma(C) uio_pdrv_genirq uio fixed i2c_dev ip_tables x_tables ipv6
Apr 11 23:31:21 allsky kernel: [22053.110362] CPU: 3 PID: 20358 Comm: kworker/3:0 Tainted: G C 5.10.17-v7+ #1403
Apr 11 23:31:21 allsky kernel: [22053.110369] Hardware name: BCM2835
Apr 11 23:31:21 allsky kernel: [22053.110386] Workqueue: events dbs_work_handler
