Can anyone advise me on a 'good' dilemma: If you publish a chapter of an unpublished novel, for example, as a short story in well known monthly magazine, would that legally hinder or prevent the publication of the novel, containing said chapter?
Thanks
Michael

Sounds like good news.

You'd need to check that you retain the copyright on that chapter - some mags default to them owning it unless you make a fuss - and that the right you grant them to publish that chapter is "non-exclusive" at least after a certain time.

If the mag's well known it's a a very Good Thing to be able to say if you're looking for a publisher - a publisher would think it was a Good Thing, not a Bad one, for you to have had that exposure.

Not to my knowledge. I remember reading a short story by Ian McEwan and then discovering an almost identical chapter in Enduring Love. The focus of the scene was entirely different but the basic framework was unchanged.