killed the brain devourer in the gaol.
didnt realise i needed to take his brain - duh!!
went back.
and guess what?
no brain...

searched high and low.
how can i get the brain?

any help gratefully received

Power Word : Reload??

If all else fails you can use the console to make it appear:
~
DebugMode 1
dm_spawnitem m1q2plotreagent <-the brain should appear where your cursor is pointing
DebugMode 0
~

thankyou Luiz.
that helped