Aussie Andy
Speaking as a private pilot in the UK, we simply plan ahead before we fly and write down the appropriate frequencies we anticipate will be required.
I asked you on a previous thread: "Where would a PPL look up the area frequency?"
Given that you fly in the UK, you may not know the Austalian answer, but I would appreciate it if you or anyone else could let me know. (This is a genuine question -I don't know if the answer is in fact as simple as it seems to be in the UK)
Also, would not having the frequncies on the chart make mid-flight changes (e.g. diversions, level changes) much simpler?