courgettes

who do courgettes when they are small go yellow and then drop off

Saffina,
This happens quite often when the courgette plant is young and hasn't made much growth, so it hasn't enough strength to grow the courgette. It shrivels and dies and falls off.
If your plant is healthy and green there should be no problem. If it's turned yellow then I would plant another.
Make sure your courgette plant is well watered and give it some kind of liquid food or fertiliser if it is in poor soil or in a pot.
The usual problem once it gets going is what to do with all the courgettes rather than not having any at all.
If you haven't already done it this year, in future years plants courgettes in earth into which you have dug lots of well rotted manure or some of the contents of your compost bin.

thanks for the answer to my previous question regarding courgettes. I have noticed that ants are in the base of the flower of the courgettes but there is no evidence of any aphids on the plant anywhere. Should I spray the ants with aphid killer or just let them be.

Just let them be. They won't do any harm and will probably help pollinate the flowers! The only problem with aphids is that they might spread a virus from plant to plant, but then many varieties today are virus resistant.