You can read more excellent thoughts from Paul Helm on ‘Natural Law’ here and here.
The first of these deals with some biblical foundations for thinking about natural law. Helm insightfuly points out, for example, that during the period when sin reigned before the law was given (Romans 5:13-14), the period from Adam to Moses, there was nevertheless an awareness among both the covenantal (e.g. Abraham) and extra-covenantal (e.g. Abimelech) characters in the Genesis narrative of moral norms which do not rest on verbal revelation.
The second picks up where the first left off and seeks to demonstrate that there is really no great conflict between natural law and common grace, rightly understood. Helm argues that Bavinck and Kuyper are responding to counter-reformation Roman Catholic treatments of natural law rather than to Aquinas and Calvin, which leads them to overstate the contrast between natural law and common grace. For Helm, the two are the same thing viewed from different angles, or at least that they overlap significantly.
As an aside, reading these its clear that Helm would not disagree with VanDrunen about natural law (he cites him again approvingly in the first article), which rather begs the question- which of them is misunderstanding Van Til, when Helm pits him against the natural law convictions both he and VanDrunen share, and VanDrunen embraces VanTilian presuppositionalism, which, apparently, Helm rejects?