Sorry, I should have said they are Dingbats, as I have never tried these before I don't know what I am supposed to do. I have worked out 14 of the 18 but these last four are just letters printed in the centre of the square.
ladyalex - after searching high and low in my PDF map of the Underground, Hillingdon was the best I had to offer, which works better if the clue was actually "gdmoundon".