I have a problem with my M4R's having just had to reset them all up again after getting a new Hub 5 from Virgin.     I have 3 floors and the top floor deco will not connect to the middle one as it used to, instead connecting to the main deco on the ground floor 

Inside the Deco app try running network optimisation, this may correct itself and connect to the middle deco.   However there is tecnically nothing wrong with it connecting directly to the main.   The deco has a system where it detects what is best for connecting to the internet, you may find that although the middle floor is closer, the signal directly to the main deco is good enough to negate any advantage of going through the middle deco.   Remember each time you "bounce" via another deco it adds more overhead and slowness to the system, it may in this occasion be more beneficial to go direct.     Is the deco performing ok?

I also get 905mbps when the Hub 5 is in Router Mode and WiFi connecting direct. I put it in Modem mode to connect to the Deco M4R's and follow all the advised ways/orders to turn it on etc and then only getting 513mbps. Both times only my phone connected (no other devices). Any advice on both would be appreciated. Thanks 

Easy one this, the Virgin Hub 5 is a WiFi 6 device (AX) whereas the Deco M4 is a WiFi5 device (AC).   The newer WiFi 6 is quite a bit faster and can get close to 1gbps, the WiFi5 being a bit older tops out around 600mbps.    The new WiFi 6 Deco range starts with X, the X60 for example.