Item bought online not as described - How to get refunded

Hi there, just a quick bit of advice please if possible. I bought a large plant from an online garden plant website which cost approx £120 including delivery. The item took 3 weeks to arrive as they sourced it from Italy and when it arrived it was different to advertised - same species but different coloured flowers (white instead of red). I spoke to their customer services who agree to get it replaced but said it would take another 3 weeks to get one. The 3 weeks came and went so I contacted them and they gave me a 2 day delivery date a week later (they said delivery on either a Friday or Saturday and they would contact me on the actual day). Delivery date came and went without further contact. It is now a week later and I have again contacted them, this time asking for a full refund as 2 months have now passed since I made the first order. Now they aren't responding to me, so anyone got any ideas what options I have to reclaim my money? I paid using a debit card so not PayPal. Thanks