Heli1
Good question regarding precedent- I do seem to recall this being done at a major airport development- a tower was built on the extended centerline; I think it was in Japan at Narita, but can't be sure of that.
Anyway, for what it's worth- the ANO refers to the requirements for an aerodrome manual publishing details of obstructions and the responsibility of the aerodrome operators to ensure the safety of operations within the ATZ ANO 128(5)- so if the obstruction is in the vicinity of the circuit (ATZ) then irrespective who's put it there- there is a responsibilites on the aerodrome operator. Also,
CAP168 Chpt 9 (5) refers -
9.5 Anything which may, in the opinion of the CAA endanger aircraft on the movement area must be removed.[I]
Now I know this refers to the movement area but I could see the same principle being applied to something in the circuit or ATZ and is mentioned in CAP168 chapter 9 (available online).
So, I think a call to the CAA Aerodrome Standards people is in order to disucss the implication of this persons actions and the potential for the obstruction to be a hazard to aircraft.