I had set nodashremux = 1 thinking I didn't need (don't use Win Media Player) but have now discovered problems with some apps (eg Avidemux). So have taken that option out.
Can I retrospectively remux affected files ?
Win7 / 3.14.0

get_iplayer has never used DASH as default for TV programmes, and it stopped using DASH as default for radio programmes in v3.14. The only way --no-dash-remux (introduced in v3.14) would be relevant is if you are explicitly requesting DASH streams. If that is the case, stop doing that. There is no reason to favour DASH. It's now strictly a backup for HLS, and --no-dash-remux is only implemented to save a little time if your machine is powered by arthritic hamsters and your media player can deal with the consequences. If you're not explicitly requesting DASH streams, then you have some other problem.

If you were in fact producing files from DASH streams, they can be re-muxed. How you do it is up to you. If you want to see how get_iplayer does it, force a DASH download with --verbose specified and look for the calls to ffmpeg in the output.

Thanks for reply and advice.
There is no reason to favour DASH ?
Serving up adequate 96 audio does save extravagance (like 317 kb/s when no hq audio requested) ?