Luke - as I understand it legal responsibility for CE marking lies with the person introducing the boat onto the market, i.e. Robson Marine. You have no contract with Robson Marine; therefore no real option for come back against them. However Robson Marine may be willing to discuss it, especially if they thought the consequence of not dealing with your concerns was a visit from trading standards
However what do you want as the satisfactory outcome? You almost certainly won't get your money back. You might get a bit of paper for CE certification purposes - but it will probably be meaningless (some people argue even the most reputable company's one is pretty worthless).
Your issue will be if you try to sell it, and people realise the paperwork isn't right. But, not meaning to be rude, but if people realise its got gemini "pedigree" thats not going to help you sell it anyway!