I also have that problem paying for vivid 200 & only geting 45 at best on a wired connection,customer services say its because i need a cat 6 cable instead of cat 5e but its been ok for years ? and it will cause me hassle to change the cable.
Cat 5e is more than capable of handling a 200Mbps connection, I use a mix of Cat5e and Cat 6 and get my full 200Mbs, so CS are just fobbing you off with that one. However it is possible for a cable to develop a fault and that can result in slower speeds, so testing with a different cable (5e or 6) is always worth a try to help narrow down the issue.
The best way to test your connection with a single Ethernet connected device is to use modem mode as in that mode only one wired device can be connected, so it rules out any slow down due to other devices.
Also check the drivers for your Ethernet card are up to date and have no reported issues. I have seen in the past folks posting on here that wired speeds dropped and it turned out to be a dodgy driver slowing them down.
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out so my answers may be wrong If my answer solves your problem please mark it as helpful as it may help others My setup: Vivid 200 Optical fibre with Hub 3 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.
No need to be sorry Matt If you can literally take out the Ethernet cable and on the devices connected to Wi-Fi take them off it also, so there is no connections running to the router and then run a test.