Looks like a couple of the WordPress plug-ins could be causing MarsEdit to fail.
Need to get it working!
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.
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.