interestingly it suggests that Heathrow is still considering whether to outline more than one option for the location of R3 in its submission to Davies.
The air quality assessments included in the planning application for the works for the end of the Cranford Agreement that you linked to are instructive.
Now that vehicle emission factors have changed following the findings that the new Euro standards are not delivering real world improvements for diesel engines, the modelled exceedances of the air quality limits for 2015 are now much, much worse than they were anticipated to be in the modelling that accompanied the previous third runway consultation in 2008.
What were narrow yellow bands over the 40ug/m3 annual limit (Fig 10.15 2015SM predicted total NO2 concentrations around Heathrow in the CERC : Air Quality Studies for Heathrow - November 2007) are now wide swathes and extend much further (e.g. down the A30 to Staines) in Fig 7.8 Annual Mean NO2 Ground level concentration (All emission sources) 2015 baseline - February 2013 in the documents submitted to Hillingdon).
Accordingly, they would need to be very creative to get a third runway in the Sipson area between the M4 and A4. It may be much easier to get something to the southwest of the airport......