I may have answered my own question. This weekend I bought a top of the range quad core laptop with a high specification graphics card.
Whilst I don't see any real difference in the playback quality of MP4 files made with the Adobe Presenter video creator tool in the new laptop versus my old laptop (both render excellent quality MP4 video files), when published to HTML from the Presenter PowerPoint plugin, the new laptop produces much better quality video. Ior example, I can clearly see the text detail in the icons in the ribbon of MS Word 2013 whereas the text in these ribbon icons looked very 'blocky' with the old machine.
So I'm pleased I may have found a solution. On the other hand, I'm wondering what the reason behind the better conversion from PowerPoint to HTML in the new laptop is? Surely, the improved graphics card can't be the reason for that can it?
After playing with Presenter more, I've come to the conclusion that the degradation in quality from the original MP4 video file (that Presenter generates with its video creator tool) to the reduced quality after importing the MP4 into PowerPoint and publishing the course to HTML is probably due to a scaling issue. I've read elsewhere on this forum and on the Camtasia forum that whenever you re-scale a video, you lose clarity. The fact is that Presenter only allows recording of the entitre screen. When you import a full screen recording into a PowerPoint slide, because the slide itself is of a size lower than full screen, the original full screen dimension in the video is scaled down.
I'm thinking that the higher specification graphics card in the new laptop I talked about in the previous post produces a higher quality MP4 video in the first place so that the effect of the scaling down when imported into PowerPoint is more acceptable.