Error Payment from Calor Gas

Browderboy

Calor Gas Uplifted tank from our new property (for free). I asked for a refund of contents £859 (their calculation) which they sent via cheque. They are now asking for the money back as say they don't give refunds for gas taken (even though presumably they resold it). Their mistake so do I need to pay it back?  They said it was paid by previous occupant (now deceased) but I said was part of the property assets. I arranged for tank uplift as executor said better for us to do as new owners. Am I legally obliged to pay back. Any advice welcome.