What speed do you get directly connected to the modem? 

If you have the R8000 from netgear, make sure qos, access control, traffic monitoring, and parental controls are all disabled. They’ll drop your speeds as they need to do traffic inspection to use those services. Sometimes a factory reset is needed and then during the install, make sure to leave those disabled. 

Also, make sure you’re testing over a wired connection. The R8000 is only going to hit that 300-400mbps over wireless 5ghz. 

Hi, Many thanks for your reply much appreciated. I just reconnected desk top direct to modem ( hard wired ) I did an Xfinity speed test, still 400 mbps.

Got a screensnip of the cable connections page and the logs?

Hi , I had a chap with ax Xfinity expert? He basically told me that the 1200 speed is there for households that have lots of occupants using lots of devices? Phones,tablets,gamers,security,cameras etc. In our home there are two of us.I’d say there are a max of 4 devices at any one time. He said that a desk top working on it’s own will never get speeds anything near to the 1200. And that 400 is all I need. He may well be correct. If this is the case am I still paying for far too much “ total’ speed? Can I drop to 800? or ever 400? will my few devices still work well? Thanks in advance. Neil