It was a screw not a nail and about an inch long. Today I inflated the tyre to as much as I can (it was leaking where the screw was), pulled it out, moved the car to and fro a bit to push the seal inside gunk into the puncture. The tyre subsequently inflated fine and I took it around the block a few times to make sure the seal inside is in the puncture hole. So far so good.
it’s magic when it works!