Thanks for your comments. I appreciate all the advice I can get, being new to all this.
If I can just appear to be totally stupid at this point, and ask the following direct question.....
Are you saying that it is only necessary to submit a given site once, for all time (or at least for a long time), and that NOT bothering to resubmit regularly will NOT mean slipping down the rankings, as others who DO submit reguarly overtake you?
Sorry to be so insistant on clarifying this, but it goes so completley against everything else I have read so far. As a newcomer it's hard to know who to trust when thirty people are telling you to submit monthly, and then someone comes along and says it makes no difference to 'dropping out' of the index, or your ranking or anything else.
This has turned my (very newly developing) world of SEO learning completely on its head!
Thanks again though.