Thank you Nev, that must be it. I saw it clearly only briefly, from the front and a bit below, I guess the beak would appear straight from that angle.
I apparently misremembered exactly the orientation of the black stripe. (This is why human witnesses are so hopeless in criminal trials. It isn't just me.)
In fairness I did look at pictures of the Olive backed sunbird but I didn't notice one with a stripe, they had more of a bib. The strong black stripe against the very bright yellow on the one I saw was very clear. Now that I search again with 'eclipse male' I can find other pictures just like yours.
I'll try to get a pic if it hangs around. I thought I heard a sunbird or two in the garden yesterday but I couldn't see one.