I have found some more good ones over the past few months. And all fairly central. One even has a key hole in it which, I suppose, means it wasn't self-locking like a lot of the others. I must find out how the coalmen opened and lifted these lids, especially the self-locking ones. Perhaps they had huge magnets or special crowbar-type things? If anyone knows please do let me know.
London coal hole cover plates here.
Click here to read Yelfy's research about Hayward Brothers.
And here are some wonderfully different and colourful coal holes from the North of England.