Thierry,
With a few precautions, I'd place the antenna on the chimney. The
primary precaution is to make sure the chimney is constructed so as
to be able to support the antenna and mast! Chimneys are not usually
built to withstand very much stress from the sides, the antenna and mast
will place higher than usual stress on the chimney because of they're
wind resistance.
The effective height above ground will be slightly lower than the
antenna's actual height because of the proximity to the structure. No
idea what that effective height might be, and probably not a huge amount
of difference from the actual height above ground. In general, the more
height, the better the performance. A meter or two won't make ~that~
much difference either way, though.
I've no experience with your particular antenna so am no help there.
Good luck...
'Doc