Also, would you not need planning consent, 5 meters is quite tall after
all.

Quite possibly. One advantage of the scaffoldling approach is that I could
argue
that the structure is temporary, and if I could not get retrospective
approval I
could if necessary sell the scaffolding.

A common misunderstanding is that just because a structure is "temporary"
that there is some relaxation in the rules regarding planning approval.
After 28 days (Occasionally 14) you still need full planning approval. If it
is not visible from outside you boundary and nobody complains then you may
get away with it.