Mini red pepper leaf drop

I live in the UK and have three mini red pepper plants growing inside on my windowsills in different pots. They are all bearing fruit, and all in the same compost. Two are fine. One looks sad :( It was fine until a bit ago, now its leaves go like so:

1) They curl up from the edges
2) Go yellowy around the tip and edges
3) The tips and edges go brown...
4) Then the leaf curls up and goes ALL brown and crispy...
5) then drops off!

Why is this happening?