As an update to this I've had time to have a proper look. It seems that the brake disk cover (is there such a thing?) on the front left behind the actual disk has crumbled and come off. There seems to be a short wire with what looks like a sensor of some kind at the end hanging loose behind the brake disk (something to do with ABS my bro thought?) which was previoulsy clipped to the unidentified crumbled metal. I've tied this to the brake cable to keep it out of the way for know.
Is this an urgent repair job?, everthing is working ok to my knowledge and I've got a lot on for the next 2 weeks but the MOT is up and the end of the month

and the Mrs will kill me if its got to into the garage again
The back wheels appear to have these covers behind the disks but the front right has never had one to my knowledge and as I've mentioned I've just lost the left one - its the dangling cable / sensor thingie that's bothering me
Sorry to have rambled - would appreciate it if anyone has any thoughts or advice - thanks