Very nice item FDS

London EC (ie: East Central) Paid, 12 April 1864.
I can't recall seeing that much of the cancel on the stamp, but small parts of the cancel are seen from time to time.
If the D of the DX is the time code, it would've been stamped between 5.30AM and 6.00AM (assuming I'm understanding my literature, but I probably need to brush up as it's been quite a while).
The letter, posted from Newcastle, left Sydney on 21 February and arrived at Marseilles on 11 April.