Interestingly, the idiot uses phrases such as 'the flying of model aeroplanes', 'the aerodrome' and 'model fliers'. This indicates a level of 'expertise' one would not normally expect from someone averse to aeromodellers' activities; I would have expected the use of denigrating words such as 'toy plane' from someone who dislikes aeromodellers in the way this idiot appears to. Also, the standard of English is surprisingly good.
To me this indicates that the writer of these letters is a calculating, dangerously unbalanced individual. He needs to be tracked down and prosecuted with the utmost urgency.
Not the ideal surprise to find coming through the letterbox in the morning. Worrying also in that any threatened damage may be applied in quite a subtle fashion.
Fingers crossed it gets no further than the two letters received by THT.