I have applied for a science PhD and am wondering if accepting it, if the offer is made, would be a mistake, as I have no scientific work experience and am afraid that merely having two degrees and no experience in a scientific occupation may render my degrees worthless. I am very aware of how I have no chance of entering a scientific job now with a BSc and no experience - is it reasonable to assume that having a BSc and a PhD and no experience would make me any better off?

I think you might be looking at this from the wrong angle. If you complete a PhD, *that* will be scientific work experience which will stand you in good stead for the future. I think the question you should ask yourself is: "Without previous work experience, do I really know what a PhD will be like?"

I think it's really really important to have some experience of what the research environment is like before you start out. A dissertation project may give you some insight, but it's not the same as working in a lab day in, day out, for an extended period.

Why do you think you have no chance of entering a scientific job with just a BSc and now work experience? I imagine that's how a lot of research technicians start out. If I were you I would contemplate working as a Research Technician or Research Assistant for a year, and then apply for a PhD. If you're worried about what to put on your application, then emphasise any general skills/experiences you have acquired in other jobs/voluntary work. Just like any other job, communication skills, teamwork and initiative are all things that have value.

Good luck! :)