Hi Jo,
It may be helpful if you could post a higher resolution image of the maker's marks. IW could be John Warner or his son, James Warner. However, when I zoom in on the right maker's mark, it looks like the top edge is serrated but it gets too pixelated for me to see properly. You can probably see this better with the naked eye or through a magnifying glass.
If a serrated box, it could be John Williams although there is some confusion here between Douglas Bennett, who attributes IW in a serrated box to John Warner while Cork Silver and Gold attributes IW in a serrated box to John Williams, as does Trev in this thread
http://www.925-1000.com/forum/viewtopic ... 56&t=31869
John Warner's usual mark has a prominent entrance serif which I don't see in your marks and may even be confused with a bullet between the I and W if worn. Bennett shows two other John Warner marks with bullets between the initials.
Seeing two makers marks and no sterling mark is not too unusual. I have a set of six teaspoons by John Humphreys (or John Hillery) with two sets of IH on each and no Sterling marks at all.
Kind regards,
Gordon