I found this data on the OFCOM website and though it might be fun to play with.
To make it usable a friend (thanks Bob) showed me an Excel utility that convert Ordnance Survey map references into lat/long and we were good to go.
The last part if the puzzle comes from Brandy Lackey who published a method of changing the mark size on a map when you zoom. Awesome.
I added to it by restricting the maximum size of the mark, otherwise they scaled too quickly and even a size 2 mark was swamping the map.
I am going to have another go at this when I get time and add a MapBox satellite map to see how that looks.