Diagnosing brake problem

[MartinQ]

I have a BMW 123d which has developed a vibration above 40 mph. I stopped to investigate and there was a burning smell and the front nearside wheel was noticeably radiating heat. I suspected a sticky caliper but I removed it and the piston moves freely. Still plenty of pad left too. I regreased the piston anyway and the back of the pads but the problem was still there. I think it must be brake related due to the heat generated and nothing looks broken or is rubbing.

Is it worth changing discs and pads and/or caliper? Anything I may have overlooked?

Thanks.

[MartinQ]

Now you mention it, I had trouble getting the outer pad back in - the protrusions(?) on the pad barely fitted in the guides on the caliper. Not sure why. But that may be the cause.

[MartinQ]

Just to update this - I couldn't find a fault with anything in particular so took it to the local garage. It was something wrong with the caliper in the end. The piston was free moving but for some reason wasn't releasing fully. He dismantled it, inspected and rebuilt it but couldn't find anything and the problem remained. A new caliper and pads were installed and all is well again. No play in the axle, suspension etc so hopefully that's solved it.

Cheers for the advice all.