Thanks for suggestions folks! Spoke to the dealer i got it from - turns out he offers a free 3mo warranty on all car sales (and i didn't even know about this!)
Anyway took it and had it checked out - there was a split on a rubber coupling on the air intake downstream from airflow sensor, so was taking in un-accounted for air.
Patched it up with superglue and duct tape, pending a replacement coupling coming on tuesday.
When i bought the car they had to replace the brake servo (i noticed a hissing noise) before i purchased. Apparently this involved moving the engine slightly off its mounts, during which i guess the coupling could have torn.
Problem solved