Dear client,  

Potentially Challenging the Demand:

There might be grounds to challenge your uncle's demand depending on the specifics of the situation. Here are some factors to consider:

Content of the Partition Deed: A well-drafted partition deed should clearly outline the specific portions allocated to each beneficiary. If the deed clearly assigns the property to your father, it strengthens your case.
Pressure to Sign Separate Agreement: If your father signed the separate agreement under duress, this could be argued in court. Evidence supporting this claim would be helpful.
Validity of Separate Agreement: Without details of the agreement, it's hard to say for sure. An agreement lacking specifics like the property's designated portion might be deemed unenforceable.

That document is not registered

un registered document is in admissible in evidence 

refuse to bow down to uncle demands 

1. As per the information mentioned in the above query, makes it clear that there has already been a partitioned of the subject property as per the partition deed.
2. The question of the person demanding his share now, stands obsolete as per the law of limitation.
3. Even if the goes by the paper then also there is very less chances to prove that the same is been signed by your father as it has not been registered, and now your father is no more.
4. Rest, you are free to contact me at any time for further discussion in the present matter as stated above.

 If you can prove beyond doubt that your deceased father had signed the registered document in favour of his brother under duress, coercion and/or threat, then you can file in the jurisdictional Court for cancellation of the deed and restoring the property to the legal heirs of your deceased father.

You need to prove that he signed in coercion pressure by filing suit 

The registered partition deed without the mention about the schedule of property may not entitle the title holder to acquire the property.

Hence he has to get a registered rectification deed to rectify the mistake 

- As per law, a settlement /Partition deed can become void if any of the requirements are not met, and further an unregistered Partition deed is not valid ,

- If your father had signed only on stamp paper , then it cannot be treated as valid deed. 

Show partition deed !