We have house and we want to sell

You have no title deed for your property though you are paying tax.  Tax-paying is not proof of the house you have.  The legal heirs shall execute the sale deed now in your favour.  Otherwise, you may file a declaratory suit in court.  Consult a local advocate with the papers of your house and tax receipts for further guidance.

The promissory note is not a title document.

If you have any other proof for having paid the consideration amount to the deceased owner earlier then you can approach the legal heirs to execute the registered sale deed on your name.

You cannot do anything if they refuse to entertain your request.

The court will not come to your rescue.

What is this Government house and private ownership? It is confusing.  Were you paying any rent to the actual owner? Are you now paying any rent to anybody? How long ago did the owner die? If she died more than 12 years ago and you are in occupation of the flat without paying any rent but paying all other dues like taxes that an owner should pay, you may be able to claim ownership due to adverse occupation for more than 12 years under the law of limitation. But unfortunately such a claim cannot be used as a sword. No court will declare you as the owner. But if the legal heirs who are abroad stake claim you can defend under the law of limitation. Otherwise now if you try to sell the property, it will be difficult to find a buyer. But if a daring buyer is willing to take the risk paying you less than the market price you can try. If the legal heirs stake claim later, the new buyer can defend himself.

Being an allottee but not titleholder of the house you have no right to sell, it is government property.