I'd like to find the original statutory source for all of the respective parameters of different section 8 eviction grounds, for assured shorthold tenancies in England & Wales.
User xngtng's answer already answers the question, but to add a little more detail:
Most of the rules associated with those grounds are contained in section 7 of the Act ("Orders for possession") and the eponymous section 8 ("Notice of proceedings for possession"). Sections 8A, 9, 9A, 10A, 11 and 12 add further details relating to particular grounds, and there are occasional references to the Schedule elsewhere in the Act too.
Section 8 of the Housing Act 1988 provides that
(2) The court shall not make an order for possession on any of the grounds in Schedule 2 to this Act unless that ground and particulars of it are specified in the notice under this section; but the grounds specified in such a notice may be altered or added to with the leave of the court.
Section 7 of the same Act also provides that
(1) The court shall not make an order for possession of a dwelling-house let on an assured tenancy except on one or more of the grounds set out in Schedule 2 to this Act; [...]