If not an inside job than it is clearly from someone who lives within earshot - ie a very close distance from Old Warden Airfield. There can scarcely be more than a couple of dozen candidates.
Surely they must have expressed their opinions via more conventional means in the past, probably several times. This is hardly likely to be their first contact with Shuttleworth.
Even by adding "insiders" it would seem a fairly small range of potential suspects, surely not too taxing even for Bedfordshire's "finest" (fnaar, fnaar) to deal with?