I had motor insurance cancelled by car insurer several days before it was due to start because they said I did not meet the underwriting criteria. They refunded the policy.

I was worried about needing to declare the cancellation to other insurers in the future which would greatly increase premiums. The insurer however told me the cancellation did not need to be declared to other insurers. The advised that the information would not be shared with any databases for other insurers, so other insurers would be unaware.

However some time later I checked the MIB MID and lo and behold the policy details were in fact registered there with the policy start date showing as the date it was originally supposed to start and ending on the same day.

I complained to the said insurance company who are now refusing to have the policy details removed from MIB despite their prior assurances that the policy details would not be shared.

Do I need to declare the cancellation to other insurers despite what the insurer is saying and also how do I get this entry in the MIB removed?