Cross-hatching on shadows with deferred Lighting

I'm having trouble working out how to avoid these dark visible seams in shadowed areas of my terrain when using deferred rendering:

Details:

• This is standard Unity terrain

• Rendering path is deferred lighting. Using vertex or forward instead doesn't have this issue

• Lightmap size is 1024, and the "squares" of shadow seem to occur every two samples. Yes, it's a low-density lightmap but it works fine using the other rendering paths. Also, doubling it to 2048 creates the same size squares - I suspect they'e based on the terrain's resolution rather than the lightmap's

Someone else must have come across this but I'm having no luck searching. Is there a setting I can change to fix this?