There is an interesting post at Richard Chappell's "Philosophy, et cetera" blog that discusses (among other things) natural law - in particular, Bentham's criticism of it. They're interesting, to be sure, but they miscarry.
1. The first criticism is that there are conflicting systems of natural law. I would have to see the research behind such a statement (knowing Bentham there probably wasn't much) before I could give a full response to this. But it does not seem so; at least, if we're talking real natural law accounts rather than the caricatures of it made by its opponents. Natural law accounts are hierarchical; i.e., laws are nested within laws. The more general the principles, the more the evident the principle and the harder it is to apply to concrete particular instances. The more particular the principle, the easier it is to apply, but the harder it is to fit properly under the general principles in which it is nested. In fact, the more general the principle, the more agreement between varying authors within the natural law tradition; the more particular the principle, the more room for disagreement. This is not a problem that is exclusive to the natural law tradition; it arises not from the notion of natural law itself, nor from any of the principles used, but from two facts: a) the more general the principle, the more you've abstracted out particular factors that need to be considered in this particular sort of ethical case; b) the more particular the principle, the more complex you'll find the set of factors that have to be considered to come up with a correct judgment. All ethical theories have to face this difficulty.
2. The second criticism is that the natural law position appears to confuse physical laws with normative laws. In fact, I think you will find this is never actually the case. Natural law is not called natural law because it is like a physical law (law of nature). Physical laws were called 'laws' because they were originally considered to be like positive laws. Natural law, however, is called natural law because it is the basis in rational natures of positive laws.
3. The third criticism is one I'm not sure I understand, because it seems an absurd objection: Nature must have had some reasons for the law; wouldn't it be "surer, shorter and more persuasive, to give us those reasons directly?" The answer I'm tempted to make is No. Actually, it's a little more complicated. To say "nature must have had some reasons for the law" is as much to say "nature must have had some reasons for ethical reasons". And so, since we're talking about ethics, the answer is this: for all but the very most general principles of natural law nature has given us the reasons; and for the most general principles of natural law, since they are the most fundamental ethical reasons, any reasons for them would not be ethical reasons but some other sort of reasons, and therefore not likely to provided a "surer, shorter, and more persuasive" way. To put it simply: nature already has given us the relevant reasons directly; the difficulties of ethics are due precisely to that fact, so Bentham doesn't know what he's talking about.
4. The fourth criticism is that, besides being false, natural law theories are pernicious, since they encourage a) dogmatism; and b) anarchy.
As to (a): I have two points. First, I would be more impressed by harangues against dogmatism if it were coming from the mouth of someone other than Bentham, who's hardly one to talk. But more importantly, I don't see any reason to think that the natural law view is more (or less) likely to lead to dogmatism than any other view. What makes people obstinately dogmatic is certainty that they are right; this is generally more a matter of ignorance than ethics, and it doesn't seem likely that we could find a view of ethics that actually made it impossible for people stubbornly to assume they are right. And indeed, utilitarianism turns out to be far worse than natural law in this regard; serious concern for natural law always forces people to reason out what their doing, and to try to show how it falls under the right principles, but utilitarianism short-circuits this by reducing it all to a means-end analysis that encourages people to think the ends justify the means. And that's exactly what fanatics do.
As to (b): I don't see that it's plausible. First, because sometimes people should rebel against unjust human laws; and the natural law position is that what makes unjust human laws unjust is their conflict with the principles of justice, i.e., natural law. Take a good example of a case where people appealed to the higher law, i.e., natural law, the civil rights movement (the abolitionist movement would work as well). Can we honestly say it would have been better for people just to sit back and accept the injustice of the laws they were faced? Second, genuine concern for natural law forces people to reason out what they are doing, as I noted previously (the civil rights and abolitionist movements provides good examples of this as well). Third, the sort of rebelling that would follow from natural law would have to be consistent with natural law, which would restrain any genuine anarchy. Fourth, the real problem is not people refusing to obey human laws because they see them as in conflict with the higher law, but rather people going along with human laws despite seeing that they conflict with the higher law. That is, people are not generally inclined to much rebellion, even when it's good for them to rebel, because people are too concerned about the consequences of their actions. They accept laws legitimizing slavery or discrimination because they are afraid of public opinion and the police. So anarchy wouldn't be a likely consequence anyway; sanctions are by and large effective, even when they shouldn't be.
So the long and the short of it is that I don't accept any of Bentham's objections to natural law. Some of my responses here are particularly focused on utilitarianism, so it's always possible that they might be evaded by someone who's not a utilitarian but is appropriating Bentham's arguments. But I suspect most objections to natural law could be met fairly easily; it would require, however, going into more specifics than I have here.
