Hi all! I thought I'd drop back in with an update, sorry it's been a while. I tried the WD40 thing to no avail so ended up taking it into a local performance car specialist to get it up on ramps.
The squeak was just one of a number of noises but the one far easiest to capture on camera. When I booked it in I said "it sounds like something's disconnected" and I wasn't far wrong. One of the lower bolts in the suspension hadn't been tightened up properly and everything was moving about. Cut off and replaced, and all the weird noises; the squeaking, the banging, the grinding, have stopped.
So, my biggest concern with the car was solved very cheaply and yesterday's drive home from the garage, through traffic and in the rain, was by far my happiest since I picked up the car.
I'm feeling much more positive about it now.