Does anyone know if filming really does take place over a 12 week period?

The weekly tasks only seem to take up 2 or 3 days max, plus a boardroom day, what do they do for the rest of the week?

No it doesn't take 12 weeks - it takes about 1 1/2 months. For this recent series, they started filming about September 24th 2012 and this week's interviews was filmed on November 3rd 2012, so make of that what you will.