IMDb lists the "intended ratio" as 1.85:1, for whatever that's worth. A friend of mine saw the film with the Philip Glass ensemble a few years ago and I believe it was shown "in widescreen" (as he described it).

Anyhow, thanks for the heads up about the IRE version. I'm going to get onto that this evening, if for no other reason than Godfrey Reggio probably had a better idea of what he was releasing than MGM. (And also so that I can hopefully realise my dream of creating a 2CD set of the "complete original soundtrack")