Tringertop

I am a British citizen currently living in Japan, where I am resident for tax purposes. This tax year I will receive interest on savings that I have with NS&I in the UK. -- Could you please let me know how much interest I can earn in the UK before I have to start paying tax on it in the UK? -- If I have to pay tax to the UK, how do I set about doing this? Can I do it online? -- I believe there is a Double Tax Treaty with Japan, so if I have to pay tax to the UK, can I claim that payment back from the UK, or avoid paying it in the first place? -- If I have to first pay it and then claim it back, could you please let me know the procedure for claiming it back, or point me to a helpful webpage? -- Is there a deadline by which I have to make a claim to have a tax payment returned? Many thanks.