You can bring enforcement action against these leaseholders as a breach of the lease covenants as they undertook alterations without consent. Once you own the property, then you would need to write to them first and request that they remove it and if they fail to do so, you can enforce it in the court.
Is it only the freeholder that could enforce this action?
If the current freeholder did not write specifically to the person in breach of lease to ask for removal, is there a time limit on getting them to take it down? or does a breach in a lease covenant have no time limit?
It would depend on the lease, but usually only the freeholder can enforce the covenants. Other flat owners may be able to do, but usually this is through the freeholder. There is a time limit of around 20 years, but if it continues with the freeholder's knowledge this could be deemed acceptance.