Is there any merit to leaving the unit to sit without it's battery fitted and powered down for a few days as I have seen suggested elsewhere before I try anything else?

You can, but Idon't hold your breath  – this would be more relevant if you were getting check sum failures or mysterious initial lockups, but these are not your issues. I only mentioned the battery because your problems seemed to be triggered by messing with it, but it may well be coincidence.