Hi Gareth,
Squeaks are notoriously odd things to sort out. Has it come back? I think the clue here is that it went away after it had been 'looked at' by the dealer. So something changed while it was there. From my experience of a zuzzing sound in a Ford mondeo when driving between 30 and 45 MPH, (a leaf stuck in the radiator, which whistled between those speeds), to knocks when cornering, which turned out to be a cricket ball under a seat rolling around, it could so easily be not related to the car itself, but to some foreign object, or something loose, which should be tight. Plastic batteriy covers are always worth checking for tightness, so my bet is they went round prodding things, which unintentionally cured the squeak. I had a woman report a loud roaring noise at speed, which disappeared when I shut her bonnet properly!
Please let us know if you ever found out the cause.
regards, John