The plot thickens on this. Was having a quiet day work-wise so thought I'd try again via the Qatar chat function.
They're saying that you can only upgrade the entire one way journey (so LHR - DOH - MLE and vice versa) not just DOH - MLE.
I posed the question that surely that means J-F upgrades are impossible for any UK based members, as LHR - DOH in F is excluded for Avios redemption.........silence.....