Mortgage Protection cancelled after payment taken. Claim on subsequent death?

There is very unlikely to be any legal issue here. There is very unlikely to be any wrongdoing by the insurance company.
Brendan I think you know that financial services consumer protection law is based on the premise that there are huge imbalances between provider and consumer! Mainly in terms of knowledge and capacity.

There is greater leniency given - both in law and in practice - to the consumer than provider. It’s possible - and I stress only possible - that the provider cancelled the policy despite having received payment. Mental and physical capacity of the policyholder are relevant as well.