No
They have to prove it
(3)It is a defence for the accused to prove—
(a)t hat he had no reason to believe that there was any person within hearing or sight who was likely to be caused harassment, alarm or distress, or
(b) that he was inside a dwelling and had no reason to believe that the words or behaviour used, or the writing, sign or other visible representation displayed, would be heard or seen by a person outside that or any other dwelling, or
(c) that his conduct was reasonable.
Its not hard to get your head around.