| You need to look at your insurance policy. Mine has a schedule describing the car insured and there is no provision that would void the policy if the registration mark was not correct. So I think you are probably insured.
However, your insurance certificate at the date you were stopped will show a different registration so you will have difficultly explaining it to the police. I think you may need to get a letter from your insurers confirming that you had cover in place at the relevant date. I do not think you will be prosecuted unless your insurers take the view that failure to inform them of the change of registration is a material fact which invalidates your policy. My own view is that this is unlikely but I know some insurers can be very unhelpful. | 
