You should write to the old management company making reference to the termination clause in the contract with a further copy of your invoice. As they failed to comply with the terms of their own contract by failing to inform you of their cessation of the management function they must be liable for the work done. If they fail to pay you - and you could say in your letter something like if they fail to pay within 28 days the matter will be referred to the small claims court for judgement. Such a case might not be acceptable for this procedure so you will need to do a bit of homework first. If accepted as a valid case, and should it go that far, my guess is that the management company wouldn't bother defending the case and will lose the case by default with a judgement in your favour. Do some googling on small claims court process before you write would be my advice.