On the surface, the London Lives petitions of the late 18th century are not that different from the early years. They used the same language of humility, the same formal structures and layouts, many of the same grievances. But during the 110 years covered, I think there are some major shifts going on in the use and meanings of the petitions presented to the Sessions of the Peace.
The growing length of petitions is significant. Long petitions tend to be from groups or institutions or high status individuals; they are often very legalistic complaints about regulations, or about breaches of regulations (they may often quote Acts of Parliament). Short petitions (apart from incomplete fragments) tend to be the least formal and their language can be subtly different from the usual conventions. Some are letters, or missing some of the formalities. Also, they seem to be more likely to be written in unpractised hands, to be untidy and use idiosyncratic spellings.
The growing proportion of parish petitions is also noteworthy. There is a very well-oiled machinery for using petitions to appeal pauper removals. They don't use pre-printed forms, but they are highly standardised. Here, petitions are being used
Combined with the growing length and formality of non-parish petitions, I think that by the 1790s a much higher proportion of petitions came from organised groups and institutions, some from high-status individuals - and far more of the petitions were drawn up by lawyers. In the early 18th century there were far more genuinely 'humble' petitioners in terms of social status; and I also suspect that closer investigation will show that more of them were women than by the end of the century.