So I replaced the sensor with the one from the original MAF and it seemed better for almost 2 hours before starting to hunt again! It did it properly twice, then for the rest of driving nudged up a tiny amount when letting off the revs.

How annoying!

The fact it got a tad better might be due to either the sensor being naff or because I removed the engine cover and reseated it, perhaps its not on properly. I'm going to investigate more over the weekend and see what I can do.