Thanks for the suggestion. I should have been explicit re audio drivers - the existing drivers haven't been updated any time recently, they're the same versions that have been in situ for some time now. Regardless, I stripped out all the audio related drivers & reinstalled them. I replaced all the current audio drivers with older versions. No joy with either effort. This is why I think the problem may be within the latest OS update - the drivers haven't changed but relevant OS files e.g. HDAudbus.sys have.
I also went deep with network card drivers as apparently many DPC latency issues have their origin in conflicts between network and audio drivers. No joy there either.