I have been happy with the service for a while now and last year I upgraded to M600 Fibre Broadband.
Sometime in the last two weeks though my speeds have dropped to 70-80mb max and the slowdown does seem to be replicated in slower real internet use (rather than just being a speedtest.net drop) when previously I could hit around 650 easily, both wired and wireless.
Rebooting the VM router does get me back to 650~ for around half an hour and then it drops back to 70-80mb.
Looks to me like your downstream power levels are too low. I've marked for staff to advise; Could be an area fault already under investigation, could be unique to your line and requiring a tech visit. If a visit is required they can book one for you, and you can avoid dealing with VM's famous telephone support.
The very helpful engineer has been and power levels have been fixed.
However the speed hasn't improved and the engineer told me he'd have to raise the issue with the network team as apparently there has been a string of problems like this happening recently in Harrogate.
I don't know if anyone on the staff here can check whether this area issue is known and being investigated - I'm still here on 75mb and paying for 650mb 🙂
If you're seeing 75 Mbps on a wired connection to a device you know is capable of more, and those speeds are still after you've tried the usual restart cure-all, then take a screenshot as evidence, do that tomorrow and the day after, and use those to invoke the VM minimum speed guarantee, which requires them to fix it in 30 days or release you from any contract minimum term without penalty.
Of course, if VM turn round and say "OK then, cancel", you either accept the sluggish performance and hope that whatever's broken heals through some natural process, or you up-sticks and take your next best ISP option.