Thanks for all the helpful advice regarding slugs. I will plant out more courgettes in pots and grow them again on the windowsill in the back bedroom.
Once they are fairly substantial in size, I shall plant them out again. I may put little cloches [from lemonade bottles] over them, to protect them.
I am hoping this will solve the problem. Grateful for all your gardening wisdom, thank you again, one and all, for the comments. I will not give up!