Well, almost 5 years later, (looking at the date on my original post) I still have the 440 and here's what happened. The leak was so slow that there was little perceptible drop in the oil level. I got rather adept at removing the filler plug and peering in the hole! The leak was from the inboard end of one of the driveshafts. Eventually the CV joint boot on the same shaft failed so I had the whole lot done at once. The gearbox was drained and refilled with the correct stuff. To date, it's been absolutely fine ever since.
So to cut a long story short, I never did find a suitable alternative oil.
Martin
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