Very interesting problem Dave.
Basically you have broken the law through no fault of your own - though you should have received your new documents a few days after the changeover - though of course, there was only a three day gap. You reasonably acted as though the insurers had followed your instructions.
Unfortunately this is a strict liability offence and so the law does not take the circumstances or your intent into account. You drove without insurance and that's that.
What I would suggest though is that you get in touch with the CPS or Procurator Fiscal and inform them of the information given to you by the insurers admitting their mistake. In the circumstances, the prosecutors may decide that pursuing a prosecution against you would not be in the public interest. Otherwise, you're done!
If so, you have a strong case for damages against the insurance company.