There are a number of good pages which explain how to enable permalinks and rewrite rules for WordPress, such as:
None of them mentioned my problem – the main apache conf for the website had the setting AllowOverride None.
Hence my .htaccess wasn’t being honoured, hence my redirects weren’t working (and resulted in 404’s). Hence i ripped a clump of my hair out.
gah. hopefully someone stumbles across this and doesn’t waste as much time as i did.