We've been out today in Eastbourne, so I didn't know what the letter on HS:MS was for today till I just got back and on to the internet. It so happened that I took a number of photos earlier, including this one, that contains an 'E':

Admittedly, I had to flip the photo to get the 'E' shape from the rails.
Just in case you are in any doubt, this is the bit that shows the 'E':

Apologies for all fellow HS:MSers that I won't be looking at your wonderful photos, because I'm limiting my internet time while here.