Thanks everyone for your thoughts, suggestions and telling me your experiences. When I first got them and gave them flubenvet for the 7 days I did see what I believe was gape worms in some of their poos. At the time I read all I could on the condition and remember reading that in cases where birds have had gape worms for a prolonged period of time they can cause pneumonia. After the 7 days of giving flubenvet, I never saw worms in their poos. I wondered at the time and have since wondered and discussed this with my vet that possibly my hen had a very bad infestation of worms and the 7 days wasn't quite enough. When my hen was on the flubenvet I did think she was improving. I can't seem to catch my poorly hen having a poo, I hope I catch her soon.
I'm not sure whether I'm doing the right thing but I have put her and her sisters on flubenvet again and I did see one of her sisters having a poo, I examined it and there were gape worms. I always clean up all the poos every day anyway but since giving them flubenvet again I have started seeing gape worms in the odd poo in the garden. My poorly hen's wheeze seems to be slightly better since being on the flubenvet for the past 3 days. I'm waiting to speak to my vet again to ask how long I should continue with the flubenvet this time.
Thanks again for taking the time to help. x