Ok - I may well have found a solution. I rebooted in safe mode with networking and no matter what I did, I couldn't cause any wireless issues. I did multiple power cycles and downloads of 2Gb at a time - no problems. Speed was back at full and I got between 5Mb/s and 6.5Mb/s download speeds - about right for wireless N.
I started up the performance monitor and watched the network data throughput - it was pretty consistent.
I then rebooted into "normal" mode and re-ran exactly the same tests in the same order. The network monitor showed burst traffic with regular breaks in transmission of a few seconds. Throughput was reduced and what took 4 minutes to copy in safe mode now took 11 minutes and the time to finish was increasing.
I cancelled the file copy, went into the conntrol panel and disabled all Bluetooth devices. I then started the file copy again and watched the graph on the performance monitor. While not as smooth as in Safe Mode, it was pretty acceptable and no dropouts.
I'm just about to power cycle the machine then do some longer term testing. Watch this space .......