@Rainmaker and others that were giving me some tips.
I managed to get Virginmedia to analyse and they found a fault in some of the configuration of my hub4 somehow (not sure what it was they didn't tell me).
anyway following that, I'm now consistently getting over 940mbps on speed test and nperf etc... uploads 53mbps.
I took the opportunity to tidy up my server "rack" (i.e. some diy rough and ready shelving!) and gave me a chance to check all the cabling and make sure everything was in order. Even via wifi now I'm getting over 800mbps which is good enough for me.
So thanks to all of you that gave me some tips and tricks.