In case it helps anyone else.
I have managed to answer my own question.
I finally decided to just recreate the HTML block.
I added a new HTML block on the page the the first one had been. added minimal content so I cold position it where I wanted it.
I saved it and went to drag it to the location I had had the missing one in.
I dragged it to the location the missing (hidden) one used to be in and pop the missing one springs out of nowhere. so it can take its place relative to the new block being dragged in.
When I let go of the new block there was the old one just waiting for me to click un-hide!
So I un-hid the old one and deleted the new one he presto back up and running.
I hope this helps some one.
Cheers,
John.