It has at least the top end of an S38B36 fitted by the look of it, although married to an M88/3 airbox.
Spoke to the vendor, who assured me that this is how it left the factory. I proposed to him that it's got the wrong engine, but he seemed reluctant to agree, and has apparently already had an offer of £18k for the car, which I suggested he should accept pronto.
So a complete S38B36 conversion (albeit with the M88 plenum) and not the original interior either. Vendor has text me again(!) and is adamant that the engine is original to the car, even though he accepts it's an S38. Odd.
It even says in the registry the engine was replaced, maybe he does not understand the obvious difference. Sure looks different from my 1985 M635. This engine will make it worth less since there is not much left original including interior. Kind of weird to see the different arrangement under the hood of a RHD.