OK Thanks for your input people, I've made some progress but not that much!
I installed the Minim Server and deleting .m3u files does work. You need to rescan the Minim server on the Qnap and then rescan on the Wiim app aswell. Then any changes made to the .m3u files are updated from the qnap to the wiim app including all the ones that are annoyingly named 00.m3u. Great. Good. There are a few caveats. The list of playlists found by the MinimServer is only make up of .m3u files held in the folder structure where all the flac files are stored. It doesn't include the very playlists I am really interested in, the ones I made in Qnaps' own Qmusic app. As explained in a previous post I can create instant playlist simply by specifying for example, 4* rated music, genre type, eg jazz or folk, and a year range. I've not seen and other app that does this so easily. (Have you?) The Qnap's own DNLA server called Media Console offers up all these playlists too and the Wiim app can see them and play them. The problem is that they are lost in the hundreds of useless empty and broken playlists that I can't delete. Why can't I delete them? Firstly I CAN delete them but I'd have to do it one by one because not all of them are empty, secondly the qnap media indexer doesn't pick up the changes unless you re-index the whole music folder and then you lose all the meta data (the entered music ratings and year info etc) used to create all the playlists.
So in summary, of all the (free) apps that have been suggested to me I have not found one that can organise you music libray and create custom playlists as well and the Qnaps own Music station 5. I've tried Minim, Jellyfin, Plex (useless on a qnap). Unfortunately once you have a problem with Music station and need to reindex you have to start again. Ofcourse you don't lose the files just your extra metadata, all the ratings you have made.
What I'm doing now. I'm continuing wiy Qmusic but as I create playlists and am exporting the music files to a separate directory of playlists. This is useful for two reasons. I have mirrored the directory to a large USB so I can update a USB drive I use in my car. Every few week I can bring in my car USB and run a backup update and hey presto I have new tunes on my car USB. Secondly this serves as an extra backup of my favourite tunes.
I will edit out all the empty .m3u files from my collection and reindex and start the process of re-rating everything again ... when I have a week to waste!!