Previous supplier final statement & refund not received after 6 months

Hi I’ve been paying bulb and my old supplier did gas & electricity. Although I’ve stopped the Dd to Ovo (prev supplier) I’ve not received a final statement 6 months after switch. Both accounts are in credit, the old supplier account in credit by £1000, has bulb completed the transfer? I’ve provided meter readings and meter photos… still no wiser. I’m ready to switch supplier again this is awful service. How do I escalate this to be sorted out please - Please help.