I also have gaps between floorboards, although I think that the biggest ones are 4-5mm rather than 10mm. Some of them are filled with something which looks a bit like cork but looks as though it was liquid before it set. Others appear to be filled wih some sort of twine.
However, my problem is that I have other floorboards which still have unfilled gaps and I don't know where to find either of these or anything similar. Help appreciated!