Actually, a small claims court cost is maximum £100 to file, but a warning letter informing SD that you will pursue a small claim case is enough for them to buckle as it's too expensive to run a court case for a big company rather than just settle. Win win.
Maybe, depends how much you've spent. My shoes £60 so clearly not worth it. They will lose more in bad publicity
Minimum £40 to lodge I believe but more importantly, you can't claim the costs back in small claims.