even if it is a single poled construction you still need planning permission above a certain height ...
i know first hand as i had a nice council man telling me the law a while ago
2 x 20 foot scaffold poles is not good
although i do remeber a 3 x 20 foot scafold poled one years ago .. had a monster 3 element beam on it ... then one night big gust of wind blew it down
found this ...
http://www.stratford.gov.uk/planning/planning-484.cfm
Ham radio aerials normally require a planning application.
http://www.rsgb.org/committees/pac.php#Need
so yes if its big ..