Update 14 June 2020 for a recent consideration of the Foster v Driscoll principle, discussed below, see Colt Technology Services v SG Global Group SRL  EWHC 1417 (Ch).
In  EWHC 2928 (Comm) Dana Gas v Deutsche Bank et al., Leggatt J treats his readers to a concise insight into islamic finance (particularly in para 10) which he needs to do to inform readers of the essence of the case. The operation essentially involves raising investment (with a view to restructuring), organised by the main agreement (of the ‘Mudarabah’ type), subject to UAE law, and supported by a purchase undertaking of the same date, subject to English law. The set-up therefore evidently is not one of dépeçage per se (this would require one and the same agreement being subject to different laws) however it comes close.
Inevitably following unfavourable market conditions, an anti-suit injunction was sought and obtained in the UAE, followed however by English proceedings which required the aint-suit to be lifted – something which Dana Gas did not succeed in as a result of shareholder opposition. The English proceedings were effectively saved from collapse by the involvement of a third party, BlackRock, who as a non-party to the UAE sharia proceedings, were not bound by the anti-suit injunction. The somewhat complicated result is that the English proceedings really can only limp along.
Dana Gas seek confirmation that the transaction is unlawful and all the relevant contractual obligations are unenforceable as a matter of UAE law. Leggatt J with neither emotion nor hesitation refers essentially to Rome I’s universal application: the Mudarabah agreement is subject to UAE law and he is happy to assume it is invalid under UAE law – hence not enforceable by an English court. See in this respect Article 10(1) Rome I.
That however leaves the viability of the purchase undertaking. (at 46) The fact in and of itself that the contract or its performance would be regarded as invalid or unlawful under the law of some other country than England (for example, a country where one of the parties is domiciled or carries on business) is generally speaking irrelevant (reference is made to Kleinwort, Sons & Co v Ungarische Baumwolle Industrie AG  2 KB 678.
At 48, Dana Gas sets out its case for unenforceability of the purchase agreement under English law. This includes reference to ordre public but also inevitably an attempt to ‘contaminate’ the purchase agreement with the Mudarabah agreement. Leggatt J justifiably turns this around: at 54: it is apparent from the purchase agreement’s terms that the risks against which the Purchase Undertaking is intended to protect the Certificateholders include the risk that the mudarabah and the transaction documents governed by UAE law will turn out to be invalid. That is why they needed to be separated. (In that respect merging the two agreements into one and applying dépeçage might give even stronger force to this argument: however I do not know whether under UAE law such construction would be acceptable).
Further arguments swept aside, the Court turns to ordre public.
Dana Gas nb had employed both ordre public and, earlier Article 9(3) Rome I: overriding mandatory law: a rare treat indeed. Relevant English precedent is Ralli Brothers: Ralli Brothers v Cia Naviera Sota y Aznar  1 KB 614: an English court will not enforce an obligation which requires a party to do something which is unlawful by the law of the country in which the act has to be done. Rome’s Article 9(3) operates in a similar context. However Dana Gas later abandoned that claim for (at 80) those rules of law are only applicable if and in so far as the obligations in question have to be performed in the UAE – quod non.
A switch was then made to ordre public, now with Foster v Driscoll  1 KB 470 as leading precedent. However, here too, it is only if a contract has as its object and intention the performance in a friendly foreign country of an act which is illegal under the law of that country that the contract will be considered (at 82 in fine) contrary to English public policy.
Conclusion: the Purchase Undertaking is valid and enforceable.
Without claiming anything near proper competence in Islamic finance law, it would seem that Dana Gas does not introduce new principles in that area. However in diligently applying conflicts analysis, Leggatt J in my view does practice a great service: he re-emphasises the need for parties clearly to identify locus implementi: the place of performance of an obligation. When obligations are marked out for a seperate lex causae, such clear identification of place of performance will insulate them from collapse.
(Handbook) of Private International Law, 2nd ed. 2016: essentially, almost every section of Chapters 2 and 3.