The Martin Lewis article says "If your policy ended over six years ago: The ‘statue of limitations’ means banks don’t need to keep records that are over six years old. However, there is no official cut-off time so if you’ve still got the paperwork, while your chances of success are a little lower with older loans, many still do successfully reclaim."
If you have no paperwork and the bank has no record then your chances of claiming successfully seem very slim to me
This bit suggests you should write to Santander. However it's unlikely they'll have any info on old GE contracts so it'll be up to you to find evidence.
"My lender's been taken over. Can I still reclaim?
Yes. In buying another company, the new owners are usually liable for its debts and for paying customers. Sometimes the liability stays with the old provider, but complain to the new firm and it'll let you know if that’s the case.
As an example, Egg's credit cards have now been taken over by Barclaycard, so Barclaycard's liable for Egg's past PPI mis-selling."