Who is responsible for fire stopping?

The person in charge of the building, not the contractor who cut the hole.

Under the Regulatory Reform (Fire Safety) Order 2005, the responsible person has to see that fire protection in the premises is kept in good order. Walls, floors and their seals are part of that.

In a block of flats that is often the landlord or the managing agent, as the lease sets out. In an office it can be the landlord of the common parts and the tenant inside the unit. Check your lease if you are unsure.

If a trade made a hole and left it open, you can ask them to put it right. Your duty is still there while you wait.

A compartmentation survey is a quick way to find out what state it is in.