Just realised I goofed!
When I was thinking about the garden shed, I assumed the worst case would be if all the power was coming from the part of the aerial one metre away. (As Foulestroupe points out, it won't be this bad.) What I forgot to allow for was that the power might be concentrated in a paticular direction. Therefore if there is anything looking like a TV aerial or a satellite dish concentrating the power into a narrow beam and pointing at the shed, it would probably not be a good idea to linger one metre in front of it.
However, from the description given it doesn't sound as if this is the case, and normally nobody would mount this type of aerial so close to the ground because they would want the beam to be clear of obstructions.
Second goof: I missed out some brackets and probably confused some people. The expression for the power density near the house should have been 100/(4 x pi x 30 x 30) watts per square metre.