From Chancery Lane Tube Station:

5 minute walk.

Turn left and take the right-hand staircase as you come out of the ticket barriers.  When you emerge, you will see a Next and a Barclay's Bank, walk in this direction (East) along Holborn, toward Holborn Circus.  You will pass a WHSmiths, Boots, and on the corner is the BT Building.  Take a left here, on Hatton Garden.  Diamond House is located on the east side of Hatton Garden, about a block and a half north of Holborn.

From Farringdon Tube Station:

5 minute walk.

Come through the ticket barriers and turn right.  Walk straight ahead, cross the lights (Farringdon Road) into Greville Street.  At the top of Greville Street, turn right into Hatton garden.  Diamond House is half a block down, on the right hand side.