Am in N Cheshire also and have still got the last of mine in the greenhouse as they're still swelling a bit. Working on the principle if they're still growing it's worth keeping them on the plants and am hoping they're OK. Mind you, most of mine have ripened anyway which is almost a shame as I wanted to try the green tomato chutney recipe posted elsewhere! Last year we had the local veg box man round to ask if we could supply him with green ones as there was shortage and loads of his customers were asking - are they actually possible to buy?

In previous years have picked the last ones when either they've stopped growing, the plants have given up or I've needed the space for something else to overwinter. Have then had good sucess with the brown paper bag / banana method but so has the abandonning them on the side with a few ripe ones method so maybe it's not that scientific and if they're going to ripen they just will!