Visitor visa extension without applying for extension

[Boyzone]

Hi,

My parents visited me on 18th March 2015 and they are going back in September. On their Passport, Immigration stamp says V17Sep2015 meaning they should leave by 17 September 2015.
But, I have a small function in my house on 21 September for which I wanted my parents to stay more for 5 days above their validity. Is it possible to do or will there be any issue after that?
What should be the valid process except applying for extension? Can anyone help me in this? I want to book for their tickets now.

Thanks,
Boyzone!

 

[Jalex23]

What should be the valid process except applying for extension?

None.

 

[Boyzone]

Thanks Jalex. What if they stay 5 more days? Will there be any issues? Anyone knowing of that?

Thanks,
Boyzone!

 

[Jalex23]

Thanks Jalex. What if they stay 5 more days? Will there be any issues? Anyone knowing of that?

Thanks,
Boyzone!

That will be overstaying. When they come back they will have to give explanations on why they overstayed. Social commitments are not a good excuse.