Need help solving a riddle.. its related to London

Sir Christopher Wren in London of old

designed buildings and structures of wonder and

awe.

Fine churches, cathedrals,

places of learning

and the centre of Londons most famous front

soor.

But one such construction stands sombre and tall;

a pilgrimage place for the grief of a nation.

The word you seek

is not written down here;

so head north, one fifth of a mile from this station

Any ideas? Its a tricky one...