I solved this before I finished typing in the thread title, but thought I'd post in case someone else has the same issue.

Despite the embarrassment and "Doh!" moment..

So, one screen has RLM with a pretty-in-black site layout. The other screen has a Firefox preview of the page, ugly in mainly default colours.

I spent a while hunting forums and so on wondering why text and background colours weren't coming through like they should.

I'd started on the post title, noticed I didn't have an open preview any more and wanted a screenshot of the two views side-by-side, but accidentally clicked the "preview" button rather than "preview on". 'Twas only when I did that that I saw a hidden breakpoint which would be under "Layout" on the tool bar.

Get rid of breakpoint, all comes back to as it should.

(Request for future development - to be able to carry style over to the RH side of a breakpoint as well as LH (or for someone to tell me how to do this so I don't have to re-do all the colour stuff again )

You do this already. You can build sites from small screen size to big screen size if you want to. If your like most your probably designing from large to small. I think of the areas between the break points more then the points themselves. I try to use the least amount of points as possible and still have the site look good in all views. I think the break point being under the tool bar and at what size that is depends on your screen resolution when your designing and if you working at full screen. You may have had a small glitch and set a break point out that far also. It would cause some confusion and it's another layer to look through when trouble shooting code. At least you found it and were able to move forward. Hope the rest of your site keeps going well.