fish Filter Broken...Refund???

George&Ade

Hi.I bought a brand new pressurised fish filter from my local garden centre 3 months ago.
One of its functions is a "backwash" facility which allows me to send the water through the unit and partially clean the sponges inside without having to completly diss-assemble it to clean them.
The first time i tried it,the little glass "spy hole" on the top of the filter cracked whilst i turned the "backwash" knob.(i applied very little pressure on this dial)
I am wondering,that,when i take it back(with the receipt of course)exactly where i stand if i ask for a full refund.
Will they try to fob me off and tell me to contact the manafacturers?
Will they try to get me to accept a credit note?(i have purchased a different model now from a neighbour...just a basic filter with very little to go wrong with it)
Maybe someone could tell me how i stand with this item please in the eyes of the law?
Forgot to say that where the "glass spy hole" has cracked,water pours out so rendering the filter useless.