You haven't said where you are - which makes it difficult to make any kind of sensible judgement about what might be going on. If you live in the south and had drought this year, and were watering artificially, that MIGHT have had some impact on the flavour. Otherwise, what's different this year in terms of growing conditions, surrounding area, change of feed, mulch, etc.

I don't know what variety. They fruit from June to November so assume they are an autumn variety which have spread from a couple of planted bushes. I have left them to their own devices except for some organic chicken droppings fertiliser. I live on the Isle of Wight.

Ah,well the drought explanation might be valid? I have noticed before that in a drought year, raspberries seem to lose a bit of their flavour - not too sure why, probably to do with take up of nutrients. Not enough sun would contribute to loss of taste and sweetness too.

Thanks I will support them better next year!