1. Dear Harmeet, once you have made an application for a visa in the same category as you previously had, you can stay in the UK until that visa application is dealt with. However, here, it was foolish to wait until the last moment to make such a visa application. You should give yourself more time in future. However now, you can stay in the UK until such times as your visa application is dealt with. However, in future, you should make your application before the old visa has run out as you are at riks of deportation once your visa has expired and you haven't applied for a fresh visa.
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3. Even though you are two days late, you still get the protection of the legislation which allows you to stay in the UK until your visa application in the same category is dealt with. So, you can stay in the UK until your visa extension application is dealt with.
4. I cannot tell you whether your application will be successful or not, as I haven't seen your application. You will have to await the outcome to see if you are granted the visa extension or not.