If you're a customer, you have nothing to worry about because all the obligations are on the seller. Also, there is no specific obligation for a seller to get certification in the first place, but the seller will face liability issues if the bike doesn't comply, and they have to also consider advertising standards and Sale of Goods Act, where stuff must be described accurately, be of suitable quality and be fit for purpose. In other words if you want to fit 500w motors in a certified 250w bike and sell them to punters, you must make it clear that they're different from the original bike and illegal to use in public places. Nobody is checking any of that, but you can get a massive problem if somebody crashes and dies on such an ebike. People will be looking for someone to blame and the police will be looking for someone to prosecute, in which case the old saying aplliess: "When the sheet starts flying, you'd better have your umbrella up".I was thinking in terms of custom built ebikes, where they change the forks or the motor for each customer. If i asked for a spec thats slightly different to the one they had tested?