Solid fill not displaying in viewport?

I'm finding a problem with solid fill, drawn in model space its fine, but viewed through a viewport and all I see is the outline of it in wireframe.

Any suggestions as to why this might be the case?

I'm not having this problem on other drawings, it's limited to this file, but a colleague has also had as well it so not my workstation. This narrows it down to this specific drawing being the problem.

As regards variables, FILL = 1, FILLMODE=1.

It is just solid objects with the problem, polylines with a width display OK as does fill text.