Cheers for the guide gringosteve! I used it as an Uninstall guide
I've seen a few people asking the same question that I had earlier so I'll give a brief answer in case anyone else looks here.
If you need to remove the inside door sill there are three little clips just like every other bit of interior trim. Gently push a flat head screwdriver (with the head protected with masking tape or something) under the sill around these areas and work it up. The clips should start to pop and you will notice more play in the sill. Then its just a case of working around the whole piece until it comes free. You may need to undo the metal bracket that the seatbelt is attached to at the bottom.
Green circles are where the clips are. The blue arrow shows where its been prised up a little already. Hope this helps anyone with similar probs!
When putting the sill back in place you will need to apply substancial force to get the clips to pop back in. They will go in but be careful not to snap anything!
ps- not my car in pic. stolen from Google.