I now know one of the reasons for the re-installed machine going slow, again. It was busy installing even more updates in the background. Last evening it suddenly went into "installing updates" mode, closing my applications and saying do not switch off. This went on for more than an hour and I left it to it and went to bed. Today it is running slow and svchost is using 60% cpu. I didn't see it until I selected "all users" stuff in task manager.
Can someone remind me of the thing you need to run to stop Win 10 installing? I seem to have mislaid it in the re-install process.