MAG's response is slightly puzzling in this case. MAN already offers connecting flights to LHR operated by two carriers. Significant expansion of frequencies on these services is unlikely following the successful WCML rail upgrade. The emergence of the beloved "I hate planes - vote for me!" tax which has slashed demand for domestic air travel also inhibits expansion prospects in the future. HS2 would further negatively impact demand for MAN-LHR. Meanwhile, STN, BOH and EMA are unlikely to offer LHR connections however many runways are constructed there.
Personally, I think there can be little argument that LHR does need an additional runway for its own reasons, though consequent benefits for MAG airports will be negligible and STN would be actively disadvantaged.
Just one other point, Skipness. In common with many 'gone native' naturalised Londoners (WOW! Am I in trouble for that!!! :-) ), you again appear seduced by the myth that only London-based jobs count as worthwhile British jobs. Not so. Long-haul services (by all carriers) from MAN support high quality BRITISH employment both directly and in the businesses attracted to our region by the availability of direct airlinks. The much maligned MEB3 (from an employment standpoint) are also big employers in the North-West. Both Etihad and Emirates operate large support call centres in the area providing BRITISH jobs which pay tax to the same exchequer as people employed at LHR. MAG itself, its sub-contractors, all the carriers which operate here (including many UK airline companies) … all are providing good jobs for BRITISH workers. Remind your colleagues that London does not have the monopoly on this. We regional types are not foreign competitors. Taxes generated by employment growth in the regions are just as useful to the exchequer as those generated by Heathrow.