For practical purposes, OP should be able to renew with his current insurer with no problem as they will be aware of the facts.
However, IMHO the OP has a rational reason to be concerned given how stiff and pernickety insurance underwriters can be at times.
I would endorse the general thrust of daveinchcape at post #7 above.
I too would regard the action of the insurers as a classically and factually indisputable cancellation of a policy.
We know that the reasons for the cancellation are essentially administrative and absolutely nothing to do with any moral hazard.
However, it is not for OP to presume that because he/she knows the facts that any future insurers will be bound to think likewise.
What would I do ?
I would point out to the present insurers the worrisome situation in to which their rigidity has placed the OP as their customer.
Amusingly, the insurers were quick enough to take the full contractual consideration [the premium].
The cancellation issue almost seems petty but these are the things that can cause headaches. :mad:
I would ask the insurers to do two things ;
1. Reinstate the original policy from the start date for and from which they were paid full contractual consideration.
2. Issue OP with a letter confirming the reason for the cancellation and that the cancellation is considered withdrawn / negated.
If this could be done it should negate any worries as it would straighten out the issue now and for the future.
Experience.
Although not on all fours with OP's case I had a similar type of general issue.
My mother wanted to move her house insurance to a new insurance company.
The new company said that one feature of the house put the risk outside the scope of their "
underwriting criteria" so they refused the application.
That refusal then became a
material fact.
Worse, the refusal was made in error as the individual dealing with it did not know their business.
I complained.
The insurance company replied that the refusal had been made in error as the house was actually well within their underwriting criteria.
The refusal of cover was formally rescinded and a letter issued to that effect.
When my mother moved the house insurance again I advised her to answer in the affirmative about the refusal and to include the explanatory letter she received. She did that and there was no problem with the next insurer.
Sorry to waffle on but I have seen some of these apparently simple matters escalate because the problem was not nailed down when it arose.