Yes indeed, patch Tuesday delivered a load of security updates following which it was back. Using the usual method I needed to force through:
Quality Update
2021-03 Cumulative Update for Windows 10 Version 20H2 for X64-based Systems (KB5000802)
Once this was installed, PC appears to be sleeping again. However a word of caution the known issues state this update can cause BSOD with certain printers.
That KB installed on its own via WU yet the computer still won't sleep. So tired of this.