Hi Mek,
It is clearly indicated by ESRC Quota, outlet/dept' decision is final. i think you should not have any doubt about it as long as you fulfil the eligibility criteria (residence status).
Anyway, some depts ask candidates to fill up the ESRC form before the selection process, but in other case, depts ask only successful candidates to fill up the ESRC form (i.e. Cambridge). I am waiting for such feedback from Cambridge.
Would please tell me what is your case.
Thanks