my top guesses would be

Not enough light
Too much fertilizer

sometimes plants that havent been planted out from their containers and have flowers get their next flowers delayed.

They are delayed because of the energy needed for existing fruit and new root growth.

If that is the case they should "catch up" eventually

Here is a link that might be useful: blossom drop

Hello hortist,
Thanks for the reply.
Perhaps they are still working on root growth. Maybe I should have removed the little tomatoes that were already on there?
They get lots of sun, however our nights have been fairly cool for this time of year and tomatoes like warm nights, plus weather has been very unsettled so I wondered if that could be a factor.
With a short growing season, I guess I'm impatient.