Northern Assurance company Calendar

I have a northern Assurance company limited of London calendar for 1918. The reason for the  glaring texture on the photos is due to the plastic wrap that's been placed on it to preserve it. The calendar is quite large and has some issues. First off it has a ink stain on the top left corner, it has water stains, a few pinholes around the borders , on the bottom center it has remains of what appears to be tape. It also appears to be yellowish with sun fade. There are also a few Frey's on the edges of the paper, and a few creases.

Any help on the estimated value of this calendar both in its current condition, and the possible value if it was in mint condition would be appreciated. thank you.