"dredging" does not include things done otherwise than in the interests of navigation, but (subject to that) includes the removal of anything forming part of or projecting from the bed of the sea or of any inland water, by whatever means it is removed and whether or not at the time of removal it is wholly or partly above water, and also includes the widening of an inland waterway in the interests of navigation;
"qualifying trade" means any trade or undertaking which, or a part of which, complies with either of the following conditions - (a) that it consists of the maintenance or improvement of the navigation of a harbour, estuary or waterway, or (b) that it is for a purpose set out in section 268(1), but, where part only of a trade or undertaking complies with paragraph (a) or (b), section 303(5) shall apply as if the part which does and the part which does not so comply were separate trades.
(2) For the purposes of this Chapter, the first relevant charge
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