Q.Lemon tree
My lemon tree blossoms seem to have cobwebbing around them, and then fall off. What can I do to treat this, please?

This sounds like Spider Mites.
You an treat the plant with Neem Oil.
Neem Oil is organic and safe for people, pets and Bees.
Here are some links with more information.
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/pests-of-lemon-trees.htm