On trying 'Delete Missing Assets' clicking on it gives no indication you have done so, eventually the splash screen disappears, whether it has finished I do not know but the consist is still there.
It is related to the session layer as creating a new layer gives a clean map.
The missing mapfile was a real pain, I followed the instructions regarding tzarc files but could not correct it using all the backups.
I am now using a cdp I saved a month ago resulting in 32 baffled programmed consists, cdp every day from now on!