I buy Tesco's 'value' bananas. (They're just as good as the regular ones but they're 'Class II' because of tiny skin blemishes). While I occasionally find that there aren't any yellow ones, I can usually find a bag where they're either fully yellow or just a bit green around the top. So, Plowter, I suggest you take a look at the 'cheapo' produce. You might well find some nice yellow bananas there, and save some money at the same time.
Many years ago, I stayed in Singapore for a few days, on my way to Oz. I went on a night-time boat tour of the harbour, which included basic food and drink. I was handed a banana which hadn't got a bit of yellow on the skin. It was totally brown, except for large black patches. Due to the humidity, I hadn't really eaten anything all day and I suddenly felt very hungry so I thought that I'd 'give it a go'. Of course, it was a banana that had been ripened on the tree and it was the most wonderful piece of fruit I've ever eaten. It was still firm but it melted in the mouth like an exquisite ice-cream, with an explosion of flavour.
Chris