Sounds like you have covered most things apart from head-lining, which is notoriuosly difficult to rid of the smell of fags.
Have you checked all the door pockets and other storage areas, lifted the back seats, cleaned out the boot, wheel-well and the boot upholstery and carpet?
The only thing remaining that I can think of is the pollen filter for the passenger area which can become contaminated with fungi or other microbes as well as the smell of the previous owner's fags. These are usually accessible from the area below the glove-box or the rain-deflector (scuttle) panel under the wind-screen accessible from under the bonnet. It should be detailed in the owner's manual.
Change the filter and buy one of those air-con cleaning cans in a motor-factors (I know Halford's sell the Comma-branded one) and follow the instructions.
Other than that, try your local valeting operation.