How long beforehand your current visa expires can you apply for a hot UK stop by visa?

I have a 2 years long UK visa and it will expire in 2 months time, I have to be within UK during that time so I cannot go back to my country to apply for the Visa. I want to apply for a 5 years long Visit Visa and I have adjectives the necessary documents and bank statements to support me. Its just the problem of the time that I brand name the application. Please let me know if you know any thing about this and provide me some links rather.
Thanks
Answers:   
The rules is 28 days before expiry, otherwise u have to write to UK High Commission/Embassy & ask them if they support your application before 28 days. Otherwise u cannot apply for a visa if ur visa is still valid for more afterwards 28 days. In your case its 2 months.


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