Metz 58 as slave - flashing, but not when shutter open

tried to use the Metz on the beach in the dark as a slave triggered by the on board flash set to 'controller'. I could see the Metz going off with my eye but the actual photo (which included the Metz in the image - fish eye lens) showed no flash and the picture was dark. if I set the on board to 'master' the scene is lit by the on board flash but the Metz is still (apparently) not syncing with the shutter.
the shutter was 1/20th
I gave up and went home...

just a thought, I think the Metz was set to ttl as well as slave (is that possible?) so the pre-flash communication might have concluded (because it could see the Metz) that the scene was really bright and not fired the Metz???

If the Metz was set to TTL instead of P-TTL then it was being triggered by the pre-flash from the camera. If you set it to P-TTL it'll ignore the pre-flash and strobe in sync with the main flash as it should. I'm sure it'll be fine.