Ok time for a quick update on this - nice weather has meant I've finally had a chance to get a good look at the adjustors at the bottom of the door.
There are 4 rubber grommets which easily prise off at the bottom of each door. The 2 nearest to the middle each give easy access to a 10mm (I think) bolt. If you loosen off each of these bolts, you can move them left or right (or in and out depending on how you look at it!) to adjust the rake of the window, front and back, so its closer or further away from the frame. You can easily see the whole window glass moving as you do it. I adjusted mine so that when closed the window was pressed almost as far as it would allow against the window frame, ensuring a very tight seal. Re-tighten the bolt and replace the grommets and then test run. Mine used to squeal quite badly at 45mph ish. Took it down the motorway at over 70 and no more noise.
Hope this helps someone👍