The third party risk is covered in any circumstance David.It seems then, from the posts so far, that unfortunately, it is probably impossible to get an S Class pedelec insured in the UK, for public, third party or product liability.
An S class bike is a motor vehicle in UK law, so if not insured, by an arrangement with the government and the insurance industry, the Motor Insurance Bureau covers those uninsured third party liabilities. All the members of the industry share the annual costs of those liability payments. Only last year I successfully advised a friend to use this service.
Here is the link to read the agreements or make a claim: MIB Uninsured claims