For example, when we went to Clava, we noticed that the sign boards indicated that some of the other (non-state maintained) sites in the vicinity did not have public access, which rather contradicted my understanding of the law and made me even less sure about what was what.
Any advice gratefully received.
Yes. Feel free. The right to roam is enshrined in Scots Law. Common sense prevailing of course, regarding gates, dangerous livestock, people's gardens, crops, the shooting season etc. Use your head and common sense. Ask if in doubt.
The "public access" at Clava I would imagine applies to the monuments under the guardianship of Historic Scotland where paths and signage are provided. On other sites nearby - on land outwith HS's guardianship - you have "the right to roam" on and access... but there may be no paths... or there might be a bull in the field... an oilseed rape field in full bloom... a grouse shoot in progress. You just have to be responsible and considerate.
The reality is.. go where you wish... wander at will... be responsible... dont leave litter... close gates... feel free
http://www.outdooraccess-scotland.com/
I hope this helps. Enjoy your next visit. Fill your boots!