It would seem this is a FMP known error. This is a recent discussion of same problem. How to setup Auto Start FMP Host File at Startup of Computer
There is a work around using applescript if you follow the other links that that discussion.
Thanks Jim, feels like I'm stumbling along a well-trodden path. I'll have a look at Applescript solution as you suggest.
As suggested elswhere, I have been able to sort this problem with a very simple Applescript that launches at start up and opens the FMP file in question. How strange that we have to have this rediculous work around for such a sophisticated and mature s/w program.