Ok, You need to be in position to provide supporting fire to pick up the bm, though you don't need to have capability to do so (you're broken or marched or have no FF)

Are you in position if you don't have LoS?

_________________
[url=http://tinyurl.com/bott2015][img]http://i62.tinypic.com/205fcow.jpg[/img][/url]

The support fire rules from the errata

Both sides may call upon support
unless the defender has been wiped
out or the attack stalled as described
above. Calling on support allows units
from other formations to attack with
their firefight value if they are within
15cm and have a line of fire to an
enemy unit directly involved in the
assault. In this case ‘directly involved’
means belonging to the attacking or
defending formation(s) and in a
position to attack. This rule represents
units from both sides that are not
directly involved in the assault lending
supporting fire when they see their
friends coming under attack. Units
from formations that are either Broken
or Marched this turn cannot lend
support.

Yep.  LoS to a targetable unit is required.  Just to highlight the bit from Meph's quote:
Calling on support allows units
from other formations to attack with
their firefight value if they are within
15cm and have a line of fire to an
enemy unit directly involved in the
assault

_________________
Neal

Oh, and I am correct in saying taht the models targeted by the support fire have to have been directly involved in the assault? i.e. rolled to either CC or FF? So I can't support fire into a formation if I've clipped it from the other side?

Yes you are.

_________________
Currently doing a plastic scenery kickstarter

Absolutely correct.

_________________
Neal

Umm, I think these answers may be at variance with earlier ones. The relevant quote is 1.12.8
Finally, any formations belonging to the losing side that were in a position to have lent support (ie, they were within 15cms of an enemy unit in the assault) receive one Blast marker each, even if they did not actually lend support. These Blast markers represent the detrimental effect on morale of seeing friends defeated in an assault.

_________________
"Play up and play the game"

Vitai lampada
Sir Hemry Newbolt

Section 1.12.6 is what I posted above and that defines "in a position to support" as

Calling on support allows units
from other formations to attack with
their firefight value if they are within
15cm and have a line of fire to an
enemy unit directly involved in the
assault

I may well be wrong here, but the RAW would seem to have two definitions; 1.12.6 defining how support fire works, while 1.12.8 defines when a BM for losing is added. After all, 1.12.8 does qualify the statement about being in position to have lent support, and does not mention LoS there. Note this also covers other situations of blocked LoS eg by a WE, and the probable situation where at least some of the broken formation ran towards the friends that were supposedly supporting them, coming into view in reality.

_________________
"Play up and play the game"

Vitai lampada
Sir Hemry Newbolt