The Metro station itself remains open until well after the last train of the night - the issue is access from the mainline station. In all due respect, if East Coast decide that they want to close the route from the Metro station onto their concourse, they have every right to do so.
If you are sending a letter, the reply will probably state that passengers who need step free access from street to platform should go to Monument.
And if they come back with that response, give me a shout and I will give them both barrels.

Write to Metro and East Coast at the same time and advise them both that you have contact them.
If you do get any response, please get in touch and I will advise on any further action.