Broadband speed dropped significantly since last week

Hi all, I’m an existing NovaTel broadband user in MK12 5PJ. Over the past week my fibre broadband speed has dropped from over 150 Mbps consistently down to around 30 Mbps. I’ve rebooted the router multiple times and checked cables but no improvement. Is anyone else experiencing similar issues recently or is it just me? Any ideas what might be causing this? Really frustrating as I rely on it for work and video calls.