I gave up in the end and just use
WM Capture. The results are surprisingly satisfactory, with quality significantly better than achieved after a digital download followed by conversion from .asf to .avi or another editable format.
Although file size is larger, I've found that by recording pop-outs at medium window size I get a very acceptable balance between quality and file volume.
In any case, I never needed perfection in the first place. I'm only trying to help an expat family member to watch favourite programmes in far-away places, not trying to run a bootleg commercial venture.