Well I've received a call from a very helpful lady at my local trading standards office. Cutting a long story short, I would only need to comply for the directives under the CE declaration if I was a business. If the sales are private, and in effect an extension of a hobby (which it is) then it doesn't need to be CE marked and really only needs to comply with "basic" standards. Here is a quote from her e-mail

The Consumer Protection Act 1987 defines a supply of goods by a person in the "course of a business" in relation to the offenses of breaching safety regulations, therefore, it could be argued that the action you propose, is not done as a business but more of a "hobby". Therefore, you would be exempt from the legislative requirements. Please note though that this an interpretation of the law and what is deemed as a 'hobby' or in course of a business is ultimately the decision of a Judge.

If you produce the product following the legal requirements or a relevant British or European Harmonised Standard (BS/EN) such as any plug supplied should comply and be labelled as such with BS1363 or complying with the essential requirements of the electrical equipment safety regulations, then it could be argued that you have taken steps of due diligence to ensure the product is safe, but also been mindful of potentially foreseeable risk hazards and design them out.
There are still gray areas such as what point does a hobby become a business, and obviously I could still find myself in trouble should someone pursue a civil case against me, but that could be the case if I was running this as a business and selling 1000's a year or an individual selling 10 !

I still want to undertake a lot more testing of my own, but I feel happy to know that I can still recover some of my costs by building and selling the remaining 5 units without breaking the law.