CM,
If Marshall's did relocate to Wethersfield I would imagine their traffic density would be low and relatively straightforward to co-ordinate with Stanstead arrivals/departures - certainly easier than Northolt/LHR due to numbers (though it may require yet another airspace grab). However, Wethersfield is, IIRC, fairly basic nowadays so there would need to be considerable infra investment which may well be beyond Marshall's means. Bentwaters / Woodbridge (if the upcoming SDSR closes it) are likely better options - though with a load of anti-flying locals - but at least other tenants to shoulder some of the costs. Still think Cranfield is favourite. Is North Weald too small?