I have the same issue, should have 150Mbps but have noticed over recent weeks varying speeds between 20 & 100 Mbps probably averaging 40Mbps, ( TN10 post code area) very unstable. Over the past two evenings the engineers have worked on my problem, I am directly cabled from PC to Router. Tonight after an hour I was informed that there is an issue that has been apparent since last October with a fix date of the 22nd of February. I observe that no faults are shown on the service status. I have requested an email on the problem but suspect will not receive. This is a shame as I have been with Virgin over many years with a great service.
Welcome to the forum and sorry to hear you're getting poor speeds. I've just checked your connection and can see that there is currently a SNR fault in the area under ticket F005082943. This is estimated to close 22/02/17. Once the fault has cleared I'd suggest posting here again to let us know if you're still seeing these speeds in case there are any issues with your hub that the fault is masking.