What is the test for a duty of care in the Tort of Negligence?

There is no clear answer to this question because the scope of the tort of negligence is so varied. Broadly there are three tests, though it remains unclear what test is most appropriate when. In Caparo Industries v Dickman, the House of Lords set out a three-fold test of foreseeability, proximity, and whether a duty would be 'fair, just, and reasonable' (ie, policy). In the same case, their Lordships also discussed the test of incrementalism, the idea that cases are decided by drawing analogies or distinctions between cases previously decided. Finally, the 'assumption of responsibility' test has grown to prominence thanks to three judgements of Lord Goff in the mid-1990s (notably Henderson v Merrett Syndicates).
One possible tactic when approaching this question is to treat the tests as alternatives. This is what the Court of Appeal have done after the confusing case of Customs and Excise Commissioners v Barclays Bank (where each of their Lordships gave contrasting judgements); see, to that extent, the Banca Nazionale v Playboy Club London case in the CA (cf the Supreme Court judgement). Possibly the better approach is to group cases into similar categories and apply the most appropriate test to that category; this appears to be what the Supreme Court expect the lower Courts to do at least in Professional Negligence cases, with the assumption of Responsibility clearly outlined by the SC as the appropriate test (see Steel v NRAM reaffmd in the SC judgement of Banca Nazionale v Playboy Club London).