Some useful material here - sounds as though it may be OK (one person's vet allegedly said not harmful) but not necessarily still effective. Your choice https://www.petforums.co.uk/threads/out-of-date-flea-treatment.432632/
Thanks Canary42 seems that it would probably not be harmful but that it possibly may not be as effective....I'll err on the side of caution and invest in some new treatment. Thanks to all those who replied - Sid.