Ignoring for a moment the fact that 3.3 million pax pa doesn't represent anything like 10% of Heathrow's traffic
This is a never ending debate which really just needs a single "LONDON - all airports - strategy" thread, but a quick note on the stats.
If those 3.3m people drive down to Heathrow, they will presumably generate an out and return leg, so that would be 6.6m pax, which is indeed very close to 10% of LHR capacity.
I presume they then teleport it back to the Midlands.
I don't agree with Mr Morris on this subject, but we converse on twitter and credit where it is due, he is passionate about his airport, not just another corporate dullard!