Looks like a couple of the WordPress plug-ins could be causing MarsEdit to fail.
Need to get it working!
UPDATE
New blog posts are working just fine – The issue appears to be with the post that were imported from Tumbler.
At least Mars Edit is sort of working now.
UPDATE:
After excellent support from Daniel at Red Sweater, it transpires that the issue was actually a security setting, server side, on SiteGround’s side of things.
All sorted now, so back to MarsEdit to not only create posts, but amend and update them too.