Just worked the chooks again. Looking at the dates, it has been a full year since I last wormed them. Despite checking their droppings carefully, I could find no sign of any worms. I measured the flubenvet out very carefully, mixing 6g with 2kg feed very carefully, so I'm confident that I got the dosage correct. I can only assume that they were worm-free which is very encouraging :)
It took some research to find a supplier. In the end, I was able to obtain flubenvet from farmandpetplace.co.uk . They were very assiduous, and phoned me before despatching as it is a medicine, to make sure that I knew how to use it and not to eat the birds during treatment. The flubenvet arrived promptly so I shall definitely use them again.