I'm not a lawyer but it would seem the best thing would be for your niece to consult a solicitor to instigate an eviction or at least get good legal advice (try citizens advice too beforehand - but don't phone them, best to call in personally).(posted 7624 days ago)Once your ex has a notice of eviction or intent to evict she can contact the local housing office with a view to rehousing.
I don't see why you are paying service charges unless there's something in the divorce agreement that says you must. The fact is the property belongs to someone else and it is their problem not yours.
The same applies to you...if you no longer have access to the property you need legal advice for the repossession of your furniture and I would think you would need receipts/prrof of purchase to pursue this.
Does your ex work? If not could she be claiming housing benefit and not passing it on to the owner (your niece). If this was the case your niece could take the matter up with the DWP.
Sorry I can't be of more help.