Cheers for that - had a blonde moment there, didn't I? Still, it was a Friday...
Hi to Dan in return!

We yield to no-one in our admiration of Google. As a marketing proposition, a website and a business model, Google remains pretty peerless. But that doesn't mean that they are free from problems - and sometimes quite serious ones.
Our current main website address is www.clearsitemarketing.co.uk but - like many companies - we bought several sound-alike and spelling-alike domains to cover the various variations of our name (principly to do with the confusion between "clearsite" and "clearsight").
We've been carefully following our own advice (and Google's) about creating quality, unique content and trying to build links through natural, legitimate means. As a result, we were ranking for a few nice niche terms and getting a bit of targeted traffic as a result. But then, along came a spider. A Google spider, to be exact.
At some point, it decided that some content that had happily lived on our main domain for months actually belonged on one of our other domains. So, for example, some content was relisted as belonging to www.clearsightmarketing.co.uk. Ouch.

As a result, the pages were no longer ranking, as that domain name has no presence or links. So much so that both of the pages listed at that domain were also made supplemental at the same time.
Annoying.
If this happens to you, then pages that are mysteriously reassigned to a domain alias will stop ranking. If those are mission-critical pages, then you can wave goodbye to revenues (fortunately, this doesn't apply in our case).
Just imagine if your high-profile Welsh Nose Flute landing page is shunted to another domain. All those visitors, hot to trot with cash at the ready are now going to Rival A and you're up the swanny.
Firstly, assuming you have a Google Webmaster account, you can log in and actually tell them which domain you'd like them to use. Apparently submitting a site map and confirming access to the domain isn't enough to avoid mistakes.
We actually did this months ago - to no effect - so don't expect miracles. Anyway - it's an option that's open to you in the interests of playing by Google's rules.

In geeky terms, '301' just means 'tell the machine looking at this page that it has been permanently moved to another location.' It's a technique most frequently used when you're launching a new site on an old domain.
In practical terms, if you're using an Apache webserver, you can use the power of the .htaccess file to repoint the incorrect domain to the right one. It's a bit technical to read up unless you're of that proclivity - but in case you are, here are the lines of code that you need:
RewriteCond %{HTTP_HOST} incorrectdomain.com$ [NC]
RewriteRule ^(.*)$ http://www.correctdomain.com/$1 [R=301]
If you don't know your way around a .htaccess file - we strongly recommend not tinkering yourself, but getting your website agency to do the work for you.
In our case, not only have the pages been moved to the wrong domain, but they've also been made 'supplemental'. There's nothing intrinsically 'bad' about supplemental pages, but it does mean that they get crawled very infrequently. In effect, they may not get checked again for many months. This means that our fancy 301 won't even get picked up and we're in a right pickle.
However, this page will be crawled within a few days of going live. By putting links in this article to the incorrect pages, the theory goes that Google will follow those links and discover that they've been moved to the proper domain name.
It sounds good on paper and, to prove that we're happy to practice what we preach, here are those links in full:
Phew!
Without messing about too much, it is pretty simple to delete the page that is getting listed on the wrong domain and just recreate it at a new URL. When the new page gets picked up.... bingo! Well, in theory, anyway.
But! If you simply copy the page content, then it will look like duplicate content to Google and very likely won't be given any rankings as a result.
We recommend you do not try this method of fixing the problem.
We all wish that getting and maintaining good rankings was just a case of having great content and doing some work to develop interest in your company. Behind the scenes, there are lots of potential pitfalls, and getting around or out of them isn't always that straightforward. A good website marketing company (hint, hint!) will be keeping an eye out on your behalf, and will know how to address them.
We'll revisit this topic in a couple of weeks and let you know whether our tactics have worked.
You should test false for the correct domain, not for an incorrect domain. Example:
RewriteCond %{HTTP_HOST} !^www.correctdomain.com$ [NC]
RewriteRule ^(.*)$ http://www.correctdomain.com/$1 [R=301]
That way you can point everything to one root and forget about it.
Dan says hi, by the way.