So pulled the rear swing arm off and greased everything. bb off and regreased, seat post lubed. Some how managed to twist the chain up so off they came as well. Cassette is nice and tight as is derailleur. Visual inspection shows jockey wheels etc are all good and no visual damage or alignment issues. Shifting is crisp.
There does feel like excessive drag of the chain through the derailleur though and sounds a little rough.
Noise is still there, only when being pedalled though possibly not as bad. Sounds/feels engagement related so I've now exhausted my limited skills so off to the LBS.