Although, strictly speaking, the Aerotel/Macrossan case on its facts concerned the interpretation of exclusions contained in sub-paragraph (c) of Article 52(2) of the EPC, the Court of Appeal's 4 step approach is plainly intended to apply to all of the exclusions in Article 52(2) - and in particular to the 'discoveries' exclusion in sub-paragraph (a) of Article 52(2). After all, the Court's discussion in paragraphs 23 to 41 of the judgment is directed generally to 'Article 52(2)' rather than only to sub-paragraph (c) of Article 52(2).
But the 4 step approach would seem to lead to the rejection of a claim for an inventive discovery along with its practical but obvious implementation. This conflicts with the principle identified by the Court of Appeal in Gale’s case  RPC 305 at 323 lines 30 to 35 and with the 'well established British idea' referred to by the Respondent’s counsel in paragraph 14 (last sentence) of the Respondent's Skeleton Argument. (The principle is also evident in Genentech's Patent  RPC 147 - for example per Dillon LJ at p.239 - 240.)
An example may illustrate.
Consider an invention comprising a discovery, namely that the heating and then rapid cooling of two certain African plant extracts (i.e. liquids) in a certain proportion produces a novel substance useful in the treatment of psoriasis, and a conventional process for heating and cooling said extract to produce said substance.
On the principle identified in Gale’s case, such an invention should be patentable - it is not a ‘discovery, as such’ as it includes the practical, albeit obvious, implementation of the discovery.
But on the Court of Appeal's 4 step approach, the invention would seem to be excluded because the process of heating and cooling liquids is well known - hence when applying Steps 2 and 3 of the 4 step approach, the 'contribution' would surely be found to lie solely in the 'discovery', which is an excluded category, and hence the invention would be found unpatentable.
The Court of Appeal alluded in passing to this issue in its Appendix to its judgment (at paragraphs 80 to 82) but did not attempt to demonstrate why its 4 step approach did not conflict with the principle above.