Bob Flowerdew explained the trick on GQT a year or two ago.
When you think they are ripe, wait another fortnight.
Taste them, and wait a bit longer.
When, eventually, they taste sweet, harvest them!
Of course, if they are not in a fruit cage, the birds will have taken them long before they are sweet!
If you leave them long enough (and water enough) the skins become sticky with sugar.
Ours are in a cage. We freeze pounds every year, and eat, thawed, as a dessert with cream or ice-cream.
