not quite avlechurch - I remember it very well.
The research was carried out by an independent opinion polling company that were commissioned by BOTH BBC North West and BBC West Midlands.
If you actually bothered to read the survey in full, rather than making assumptions, you would realise that is was not a self selecting opinion poll but rather that the polling company went out around ALL the regions and asked people specific questions.
The results for all the regions, not just the NW are available on the link provided earlier.
Not to sure how there was any bias or Mancs all piled into rig the result could be implied from reading the survey itself.