While I agree that a CMS and online shop won't necessarily lead to an increase in sales, I'm a little surprised that your experimentation led to a decrease. Can you be certain, as your comment seems to suggest, that your decrease in sales was connected to your experimenting with them? I'd be doubtful.
A CMS isn't necessary to make a site more presentable or user friendly - this can be managed without one - but it does help when the site becomes large and unweildy, and can help prevent it becoming a sprawling unmaintainable mess; at which point it can lose some user friendliness because it becomes hard to navigate or find things. This, I think, is the problem with RISCOS Ltd's website.
To be brutally honest, I think the same can be said for yours (sorry!). It's inconsistent, both in look and feel on different pages, and in navigation methods - and indeed in the actual location of pages, given that AFAICS only the frameset is loaded via the domain name, and all the pages under that from arcsvs.demon.co.uk
For instance, I pop along now to [link] and I get your home page. (This uses frames, as I said above, so if I found something via Google, I may end up with one of your frames as my full browser window with an arcsvs.demon.co.uk address) and I have a basic navigation frame down the left, and a main one taking up most of the window.
In that main window, I click on "Internet Suite" - this replaces the main frame with one about your DialUp4. Fair enough. If I click "Order now" this then puts [link] in the main frame, with a different coloured version of the same texture, which to my eye looks a bit odd, but never mind.
Going back to the main frame, I can click on NetFetch3; this behaves the same way, loading the relevant arsvcs.demon.co.uk page into the main frame - though with a completely different texture, which looks even more odd than the different coloured version of the same one.
Back to the main frame, and the WebsterXL link opens a replaces the frameset with a different one. Different colour scheme, now showing arscvs in the URL bar, etc.
Back again. Messenger Pro 4; New window, plain white background, etc.
Back again, and click on "Grape Vine" under internet tools. This opens a new window and a completely different colour scheme. The order button on that page takes me to [link] - the same as filled the frame above after clicking "Order now" for the internet suite, but now it's in its own window, and other than the title bar and copyright footer, there is little there to identify the page as yours.
Okay, "rcomp" appears in the URL, but as a subdirectory of arcsvs.demon.co.uk - for all I know, I could be on a phishing site, trying to rip off your customers. (Not least because the ordering page doesn't appear to be secure.)
Go back to the main framed window again, and click on Siteseer. This time the page is opened in the frame.
And so on. And so forth.
I would suggest the following things:
1. Use the domain properly, and don't pick your pages up from your Demon site. It just looks that much more professional if all the pages are loaded from rcomp.co.uk
2. Make the pages consistent, and bearing in mind that framed pages might end up opened in their own right (eg via searches, or from Adjust clicking) put a standard header/footer section on them which identifies them as yours. (Consider using server side includes for this, or if they're unavailable to you, you can download a free copy of the 26 bit version of WebChange which allows you to achieve a similar effect)
3. While I don't object to the use of frames per se, consider the flaws they present - if you must use them, at least use them correctly and consistently.
4. Secure that ordering page!
Again, sorry for pointing all this out, but somewhere down the line it has to be said by someone!