Probably down to lack of pollinating insects. You could try using a child's paint brush and hand pollinating - that's what we do with early flowering spring bulbs in the bulb house.

Run your hands over the plants causing them to shake, watering with a fine rose over all the flowers helps to spread the pollen as well....

Thank you to both of you. I'll try Lincslass's remedy first as I'm not sure I have the patience for a paintbrush.

It's true that we don't as yet have that many bees in this garden but in the last year since we moved in I've planted a lot of bee and butterfly plants so hopefully that will do the trick.

Lets hope so, I too have added lots of bee and butterfly friendly plants in the last few years and it seems to be helping, just cannot seem to get the ladybirds back...

Hoverflies also pollinate plants. Years ago a rabbit's foot
was used. People dont go rabbiting nowadays, and supermarkets dont sell their feet !

Well certainly not with the fur on Diane!