Is that the official combined MPG provided by Audi? If so, it's quite common for those MPG figures to be a little off across all makes not just Audi - although 35mpg does seem a bit low. I would expect more like low 40's...
Those figures are produced in a lab which doesn't really replicate normal driving conditions.
As for your warranty, technically the supplying garage is liable for any faults that were there at the time of sale for 6 months according to the law. Sounds like they're trying to sort it for you though?