Yes I have heard of this insurance before. However, I have owned a leasehold flat with a shared freeholder arrangement (all flats had an equal share in the freehold, there was no separate freeholder or management company) and we all had 'normal' buildings and contents insurance. Contingent insurance was never mentioned by my solicitor....
Contingent buildings insurance is a type of insurance designed to provide protection in the event that the freeholder or landlord fails to insure the building, or if their insurance policy is inadequate or does not provide sufficient coverage.
In many cases, the freeholder or landlord will be responsible for insuring the building, and this insurance will typically cover the entire building, including all individual units or apartments within the building. However, there are situations where the freeholder or landlord may fail to insure the building or may have inadequate insurance coverage, leaving leaseholders responsible for insuring their own property.
Contingent buildings insurance is designed to provide protection in these situations, by providing coverage for the structure of the building, as well as any fixtures and fittings within the individual units. This can include things such as the walls, floors, and ceilings, as well as any built-in appliances or fittings such as kitchen cabinets or bathroom fixtures.
So in short, contingent insurance does seem to cover the same things as other buildings and contents insurance for other types of properties but is specific to the leasehold / freeholder nature of the type of building you are intending to buy.