Yellow Jack.

Martello Tower, Seaford.

This was at the Martello Tower (small defensive forts that were built across the British Empire during the 19th century, from the time of the French Revolutionary Wars onwards) at Seaford, East Sussex. Interesting!

As a post script to this post an old naval friend contacted me with a bit more information:

“It was flown first in the West Indies when the place was full of yellow fever by the Royal Navy but now it’s flown for other contagious disease.”