I rang Virgin Media to request an Engineer come to move my router. I was advised to ring early morning so I rang at 8-30am and got straight through to an India and a Customer Services Call Center.
The Engineer came on 19th Feb and moved my router with a much longer Coaxial Cable. He did an excellent job and balanced all the levels. He did tell me that some building materials can significantly affect the wifi signal at various times of the day, even the type of ceiling plasterboard.
He recommended using a cabled connection into the router where possible and always keep the number of wifi-connected items to a minimum. This has proved excellent advice as I now get full wifi signals almost all day. I do have a number of devices connected directly into the router, or use mains TP connectors for others.