CFB yellowing leaves

My Hunter CFBs have yellowing and dying leaves.  The newer growth at the top of the support is green.  Also another variety is doing the same, although not so drastically.  They have been watered regularly  and fed haphazardly.  Is this a natural progression or am I missing a trick.  Other CFB are OK with no yellow leaves.

Mine's the same, JJB! I thought it was because I'd watered them early on a hot day and the leaves had burned, but maybe not. It's not because they're old, either, since I didn't sow them until early June -  - they're flowering and hopefully setting fruit - but our soil is quite poor so even with a good layer of compost on top that might not have been enough.

As per Mikey's tip about feeding, I'm going to sprinkle some alpaca poo around them to give them a boost!