advice needed

hi,
any helpful advice appreciated


last weekend I bought for £60 a 6 pack of dog flea treatment and duly used the 1st of 6 on my dog when I returned home,
within the store it was locked up in a glass cabinet and I had to leave my name/address etc
the problem is as follows this flea treatment is meant to be used every 6 weeks so it would take up until june july 2019 to fully use up all 6,but the expiry date on box is march 2019
I called manufacturers and they advised to return to store as the product should no way be used after march 2019.
I only noticed expiry date after use of 1st treatment - I haven't contacted store yet as i expect they'll fob me off and i would like to know my rights prior to engaging in any arguments


any help appreciated