Help with a dying Chilli plant please.

[Philthy]

I'd concur with Scarlet. If it's three years old already, there's a good chance it's on the way out, and nothing much you can do to prevent this. Its frustrating especially when you've invested a bit of time cultivating a strong, seemingly healthy plant over several seasons, but they are fairly short lived plants in our climate. Keep an eye on the green stems for signs of mottling/discolouration. If you see that, it means rotting is just around the corner. Keep the plant for now, but I wouldn't spend any time on it. Focus your time on the new seedlings.