The government announcement is at http://www.dft.gov.uk/pgr/rail/pi/highspeedrail/londoneuston/ and it includes this plan of Euston, showing it expanded westwards, absorbing the whole of the next block, gardens and so on.
The King's Cross Railway Lands Group is not actively involved in community engagement with this proposal. When we find out who is active we'll say so here.
There is some discussion, and a better map. at http://londonist.com/2010/03/hs2_has_terminal_consequences_aroun.php
If you have suggestions, please send them in to us at firstname.lastname@example.org
Campaigning centres partly on the West Euston Partnership. Linked here
Diana Shelley adds: You might want to send people some links to HS2 campaigning groups as well:
(that's Primrose Hill based, but part of the coalition of Camden groups)
(good on business case)
Stop HS2 http://stophs2.org
(the Chiltern-based campaigners)