I remember someone posting a little while back (can't find the thread) about having a problem with brakes squealing especially when turning a corner with no brakes applied. Well mine had started doing this, would squeal when I turned left, applying the brakes did reduce or eliminate this. On the previous thread it mentioned that it may well be caused by the lip that builds up on discs with some wear on them. Figuring this may be the problem and as Vaux were doing discs and pads for £45 through the trade club I decided to just replace them. Fitted them last night, plenty of copper grease on the backs of the pads etc. Got in the car to go to Asda, fine for about 2 miles then went round a left hander and it started again. Over the next 2 miles it got progressively worse until the squeal was constant and even applying the brakes didn't stop it. Going round a right hand bend it did reduce considerably.
Got home tonight and took the wheels off to have a look and found the problem. Did the brakes in poor light last night so didn't notice but there is a thinnish metal shield/plate that sits between the bottom ball joint and the disc which I assume is designed to remove debris from the disc before it gets round to the caliper. The gap between the ball joint and disc is pretty small anyway and basically muck from the road has got between the ball joint and the shield and forced it closer to the disc until it makes contact. This explains the variation in squealing depending on steering lock because the shield moves around the ball joint as you turn the wheel.
Simply cleared the bit of muck out and pulled the shield off the disc with my fingers (easy to bend) and no squealing Obviously changing the discs made the problem worse as the new discs are that bit thicker so made more contact with the shield. Hope this helps out anybody suffering the same problem.