Front NS brake binding Skoda Fabia 1.4 mpi estate 2001

Can anyone tell me what causes a brake to bind. I have established that it is not the piston on the callipers. One of the seals around the top pin that holds the calliper in place looks perished all the others look ok.

The pads look new as I've checked both sides to see what's wrong.

I have checked the wheel bearing and it is fine. No problem.  Is it possible for the back of the disk to be warped as it looks like its not sitting right on the hub compared to the other as there is a gap on the NS calliper behind the disk I can get my  finger in.

I have took it to a garage and they took it all apart and gave it me back exactly the same without charge.

My brother who is quite good with this sort of thing seems to think that it could be a warped disk, as its not sitting correctly he did say that it could be something else but I've forgotten what he said something about something sticking out behind it.

Any suggestions would be great.